There's only four Pokemon in Underused that I believe are worthy of suspect testing, those being Mew, Heracross, Chandelure, and Raikou.
With Mew comes the bar of unpredictability, as it can effectively run 3 very different sets, Nasty Plot, Swords Dance, and Stallbreaker. Typically, in order to find out which set is running, you have to allow Mew to set up one of its boosting moves or Stealth Rock, which puts any player in a hole. Provided you can determine what set it is running beforehand, beating that specific set is no easy task either, with only 13 Pokemon in the tier naturally outspeeding it, and another 4 tying it. The coverage on the two offensive set also limits as to what can be used to check Mew, as very few of the Pokemon that can have the luxury of being able to afford to take a +2 hit. I remember a time not that long ago when Kingdra was considered unpredictable and only had a select number of counters. Unlike Kingdra, Mew has no guaranteed counters, as no Pokemon in existence can handle all three sets at once. Its superb coverage, great balanced stats, and sheer unpredictability make it too much for the Underused tier.
Heracross defines BW2 UnderUsed to such a degree that the metagame actually revolves around teams having a way or multiple ways to exclusively stop this Pokemon. The fact that Pokemon such as Gligar are even depended on is Heracross' doing. It's an excellent and cool Pokemon, but its great offensive typing, stats, abilities, and movepool make the metagame too unhealthy, as its limited number of offensive checks can be quickly eliminated by one of its coverage moves. Heracross has become so influential that anything slower than it should not be running a Choice Scarf, as it only adds to the number of Pokemon that it can blow past and accumulate Moxie boosts. I've experienced and heard stories of players playing terribly only to have Moxie Heracross dig them out of the hole and pull a win out from thin air, which shouldn't happen. If you've played terribly, there should be no "get out of jail free" card. Take the loss you'd typically get without Heracross and learn from your mistakes.
Chandelure is another one of those absurdly-powerful Pokemon found in the tier that has a very limited number of checks. It's too powerful really, since opting against running a Snorlax, Porygon2, Houndoom, or one of its other select checks is near ladder-suicide. The only way to afely dispose of Chandelure is to Pursuit-trap it with Snorlax or Choice Scarf Krookodile, or just overpower it on the revenge kill, which still leaves it room to switch out. Its base 145 Special Attack stat is simply too high for the tier to handle, especially when many of the Pokemon that resist its STAB combination are decimated by its coverage moves. With its hidden ability Shadow Tag looming around the corner, I feel that it's in the best interest for the UnderUsed tier to get rid or consider getting rid of this nuke before the insta-ban is mandatory.
Raikou is one of those commonly-debated UU Pokemon notorious for destroying teams if they lack a Rhyperior, Swampert, Snorlax, or Porygon2. As discussed on the Viability Thread, Raikou is one of those Pokemon you must prepare for, as it's too strong to be overlooked, and will take your team to town if you don't. Its two most-effective sets, SubSM and Scarf, have very little impunity regarding what they do, as SubCM can set up on most-every defensive check that lacks a phazing move or Earthquake, while the Scarf set is the best revenge killing set available to the tier, being able to eliminate many a threat from the opponent's team. It, like the other three I've mentioned, is among an elite group in this tier that every team must have a check for, and contributes to the "lack of creativity" to say the least, that UnderUsed is beginning to fall into.
Thanks to these four, teambuilding has become rather restriced, with a certain formula needed to succeed. With these four up for suspect, and even out of the metagame, more underrated threats, such as NP Cofagrigus, Rotom-H, and Mienshao will be able to take to the stage and lead the charge on a more diverse UnderUsed tier.