As much as I'd like to agree with the other members on Chandelure being a positive aspect of the Underused metagame, I'm going to have to stray a bit from the popular opinion and flat out say that Chandelure is unhealthy for the Underused metagame, even ban-worthy, but not broken. Warning, my argument's a bit lengthy and essay-ish.
First off, I want to build my argument off the nature of the tier. In an offensive tier such as Underused, defense isn't a team's primary thought when it comes to teambuilding (outside of bulky offense, which only accounts for, assuming balance runs a Chandelure check, less than half the teams in the tier). You want Pokemon that can punch through each others checks as quickly and easily as possible, and win matches in similar fashion. This being said, Pokemon like Snorlax and Porygon2 don't quite make offensive teams (besides like, CB Snorlax). Teams that fail to run either of these two Pokemon or a Water-type with a heavy HP and SpD investment (not a common characteristic of run-of-the-mill offensive teams either) will be ran-over by Chandelure, as they have nothing that can freely switch-in on Chandelure, and those monstrous Fire Blasts and Shadow Balls. Because of Underused being a more offensively-orientated tier, Chandelure is exceptionally unhealthy for the Underused metagame, as it forces that same metagame to adapt defensively to a single attacker instead of focusing more on direct offense.
Secondly, Chandelure in itself is a bit of an anomaly. It ties for 10th in regards to the highest Special Attack stat in the
entire game (everything else on that top-10 list is currently residing in OU or Ubers), has a typing that gives it two immunities (three with Flash Fire, but Flame Body is better anyways), 6 resistances, and 5 moderately-common weaknesses, and sports a stat pool that allows it to keep pace with the majority of the metagame while being capable of taking weak or resisted hits. Chandelure also sorts a movepool that allows it to bypass many common types that would otherwise check Chandelure, such as Energy Ball for Water-, Rock-, and Ground-types, or the versatile Hidden Power (Fighting), which allows it to punch through Dark-types (albeit Houndoom is the only one this does work on, the others just pitifully die to Fire Blast). Something with such offensive utility doesn't entirely belong in Underused, a tier where overly-powerful (offensively or defensively) and versatile Pokemon don't exist in abundance, as there's not a lot in the tier capable of handling everything Chandelure does. Chandelure's overall utility, backed by an absurd Special Attack, makes it unhealthy for the metagame, as few Pokemon are properly equipped for something of its typing and stat spread.
The next point I want to build upon is the go-to checks of Chandelure and how they really don't check Chandelure and Chandelure
Teams. On the odd occasion, an offensive team may forfeit one of its hitters to make room for something like SpD Snorlax, Porygon2, or Umbreon for insurance against Chandelure, but we mustn't forget that Chandelure isn't something that operates alone. Heracross has a teammate deviation of +5.093%, and also happens to be a rather viable check to all three of those Pokemon. Smart play alone through some double switches will quickly wear the specially-defensive titans of UU down, especially if there's any form of hazards on the opposition's side of the field. Forcing Snorlax to Rest is what will hurt it most in the long run as it's basically a sitting target for 2 turns. Blastoise, a relatively common partner to Chandelure, can utilize Toxic to help break through Porygon2, who cannot heal itself of status without a teammember running Heal Bell or Aromatherapy. Umbreon's reliance on low-PP moves (Wish and Heal Bell) to keep healthy prevents it from lasting overly long in a match against an offensive team, especially since those healing moves may be directed at other members of its team. The shortcoming here is that even with one of those three Pokemon, offensive teams that run Chandelure have more than one way to get through Snorlax, Porygon2, or Umbreon, and will still see Chandelure pull out on top, whether it be from status support, hazards, a Fighting-type teammate, or a combination of the three. Snorlax, Porygon2, and Umbreon (the former two moreso than Umbreon) cannot be considered guaranteed checks to Chandelure either, as the more common variants (see Choice Band Snorlax and Tank Porygon2) don't stomach hits from Hidden Power Fighting and Fire Blast, respectively, as well as their defensive counterparts do and still suffer the same effects of Trick, Chandelure's "trump card". Choiced Chandelure (>65% usage amongst all Chandelure variants, whether it be Specs or Scarf) has notoriously made a habit out of carrying Trick around, neutering the chances Snorlax and Porygon2 have of walling it (and its teammates) while severely limiting what Umbreon can and cannot do to Chandelure and itself (healing-wise). Umbreon may be a full-on stop to Chandelure, but it has next to no offensive viability, finding its home mostly on Stall teams, where even then, it's still not that good.
My third point dawns some light on there being so few offensive checks to Chandelure. The only sure-fire offensive checks to Chandelure are those that can outspeed its Choice Scarf set while being capable of accurately hitting and KOing Chandelure, typically on a revenge kill or double switch. Five Pokemon; Darmanitan, Kingdra, Heracross, Sharpedo, and Flygon, are the names I come up with, as they all commonly run sets faster or capable of being faster than Chandelure, while carrying a STAB or coverage move that will knock Chandelure out 100% of the time, while also having relevance to the Underused tier. Something with less than ≤10% usage cannot be considered a "sure-fire" offensive check, simply because of how scarce it is and how it may not perform as expected (see kokoloko's article,
Underused 101, where he discusses in Step One, Fourth Priority: Miscellaneous
Common Dangerous Threats. Uncommon checks should not be focused on as heavily as common ones, etc.). With only 8.6% of the tier capable of safely and efficiently revenge killing Chandelure, it's more than safe to say that Chandelure is simply too good for the Underused metagame, being an unhealthy component that more than often forces you to take additional damage midst-revenge kill, potentially opening holes for its teammembers to overwhelm the opposing team.
I also want to note that predicting a switch to trap or wall Chandelure is relatively easy to do, as many players almost "fear" it and what it can do to a team.
Another point I'd like to expand on is that Chandelure is already a huge pain in the ass for many teams on the ladder and in tournaments, and it's only going to get worse. Shadow Tag is bound to be released sometime in the (hopefully) not-too-distant future (I've heard many speculations placing the release in June/July 2013) and it's going to turn Chandelure into an overpowered Gothitelle with a few more resistances and much better offensive typing. I feel it's in our best interests to just let go of Chandelure while "on a high note" and do what's best for a tier already having moderate difficulties handling it. In the article,
Characteristics of a Desirable Metagame, DougJustDoug highlights key areas in policy, such as Player Skill, Balance, and Variety. Based on the negative impact Chandelure has upon the Underused metagame, it can be seen as apparent that Chandelure being a member of the Underused tier is disrupting Player Skill similar in fashion to how Chansey did, but not nearly as severe, and creating hiccups in the balance and variety of the metagame itself. Chandelure is a potent weapon that can almost be thrown around extremely easily despite its obvious flaws (SR weakness most importantly), as it's borderline guaranteed to nab an OHKO or 2HKO (depending on the set) on any incoming or standing Pokemon, the exceptions being the three defensive checks to Chandelure and the 5 offensive checks to Chandelure - a total of 8 Pokemon (13.8% of the entire tier). With the lack of checks, players can simpy "button mash" on either Fire Blast or Shadow Ball and blindly utilize Chandelure, as its power lets it get away with it for the most part. This is the definition of lack of skill caused by Chandelure, as its power, dual-STAB, and useful coverage moves allow a player to almost play dumb with Chandelure without suffering too many repercussions. Because of this power, the metagame is basically cornered into running one or more of those eight Pokemon referenced throughout this argument to prevent being demolished by a Chandelure team. Such imbalance and lack of variety is unhealthy for the metagame, as it does hinder the offensive nature of the tier. Nothing's saying that common sense cannot be applied to policy either. Would the tier be better off without Chandelure? Easily. Chandelure holds a lot of offensive pressure, and not running one or more of those 8 referenced Pokemon can and will cost you matches. How much would the departure of Chandelure affect the tier? I cannot answer that directly per-say, but I can speculate that other Special Sweepers would finally see better usage with Chandelure removed, promoting increased diversity throughout the tier. Would something from the depths of RU or even NU be able to step foot in UU with Chandelure out of the picture? Who knows? Nothing's saying it couldn't happen. Things like Slowking may actually become more than just viable, as it won't have to worry about an incredulously powerful Shadow Ball waiting around the corner.
However, I do agree with users such as JabbaTheGriffin, who present the argument that Chandelure isn't what we should be looking at if we want to slow the tier down (although I do believe it doesn't hurt to put things like Chandelure in the spotlight)
for the time being. With Shadow Tag, Chandelure
will need to go.
TL;DR (sorry about it being so long really), ban it, but not because it's broken, but because the metagame would be better off without it, as it truly is a very unhealthy aspect of the metagame, having very few reliable checks and even fewer switch-ins. Chandelure isn't the heart attack, it's the cholesterol.