RSE Lotad/Seedot
Let's start with Lotad. Catch it around Lv3. Movepool is Astonish and Growl. This immediately presents a problem: what can it fight in the early game? Most of the early mons resist it (Poochyena) or are immune (Zigzagoon, Whismur) to it. You pretty much need to fight it against the bugs (Wurmple and Nincada). New Problem: You're weak to their STAB (Poison Sting and Leech Life). And they actually do more damage than you because they have higher attack stats and STAB, so just FU. Fortunately, the Bullet Seed TM redeems this problem, but you're left with a 20-50 BP STAB move.
And you're going to be stuck with Bullet Seed for a while. You pick up Absorb at Lv7, which is weaker than Bullet Seed. Then Lotad learns its next STAB at LEVEL 43 for Mega Drain! So yeah, you're not doing that. So you take the evolution into Lombre without delay, get a minor stat boost (still only slightly stronger than your first stage starter - 340 BST). And new problem: Lombre doesn't learn a STAB until Hydro Pump at LEVEL 49! So yeah, that sucks.
Essentially, the midgame here boils down to carrying around Bullet Seed for the majority of the game as your BEST STAB attack until you pick up Giga Drain around Mt Pyre. And you never get to use your Water STAB until you pick up Surf. Like wtf? You're in the late game now and it's finally getting STAB?!
And basically, from Brawly onwards, you're carrying around this 340 BST Pokemon. How long are you carrying it around for? Well, until you get Dive. Your only sources of Water Stones are the Abandoned Ship and Blue Shards (wild Clamperl). Both require Dive to access the stone/shard. So you're carrying this thing around. You want to know what has a similar BST to this deadweight Lombre? Spoink (330), Aron (330), Tentacool (335), Cacnea (335), Koffing (340) and Rhyhorn (345). And you want to know the best part about all of these: Most of them have already evolved! But no, Lombre requires that you lug it around from, basically, Roxanne to Tate/Liza with its awful BST just so you can finally use Ludicolo.
And how's Ludicolo? Well, actually not bad. Still has an awful movepool, but Surf/Giga Drain is the best you're going to get anyways so might as well roll with it.
"But I played Ruby so I got Seedot!" I don't think you have anything to get excited about! Swap out Astonish for Bide and NEVER get an offensive stab move in your first form. You know things are going to get WILD when you need to rely on the Bullet Seed TM to even DEAL DAMAGE! And you're going to be relying on Nature Power too, and, at this point of the game, it's mostly Swift (indoors) and Stun Spore (Grass), with the occasional Shadow Ball (Cave).
So let's evolve this nut and see what happens. Well, you finally get Dark-STAB... at Level 31, and it's Faint Attack (which is okay I guess). That's still basically going to require you to go to Norman/Winona with Bullet Seed as your only STAB. Thankfully, you get access to Shiftry right around when you get Faint Attack, as the Leaf Stone is on Route 119 and only needs Surf. After that, you basically get a Pokemon now. But, like Ludicolo, Shiftry is going to need that Giga Drain TM if you want it to finally ditch Bullet Seed.
Ruby players get basically an equivalent end result between Shiftry and Ludicolo, but Ruby players basically get it 10 levels earlier. But, Seedot fans are also so lucky that Bullet Seed is a TM... I don't want to imagine using it without it.
This so much.
Additionally, you get it all over again when you try using them in ORAS. "It has been three generations, and a lot of Pokémon have got useful upgrades, surely these Pokémon are better in this game?"
Lotad starts off well: Absorb at level 6 and Bubble at 9. Dual STAB right out of the gate, that's nice, even if the moves have really low power. Then Lotad evolves at level 14, and ... oh, you should have held on until 18 to get Mega Drain. Lombre doesn't get it at all. Oh well, it's only a 40 BP move, and at least you have ... Natural Gift from level 12 on. Looking it up, it actually provides quite a bit of power, provided you have berries to throw away. In-game, however, you won't be likely to have an infinite supply of berries at your disposal, so Natural Gift is a non-move.
Anyway, your Lombre can still amble along. Its only Grass STAB has a base power of 20, but just pick up Bullet Seed, and ... oh. Wait. No Bullet Seed TM in ORAS. Absorb it is, then! You get Grass Knot in Fortree and Energy Ball in the Safari Zone, but for half the game you might as well forget being a Grass-type. At least there's Bubble Beam from level 24 on, but 65 BP off 60 SpA isn't going to punch any holes anytime soon.
At least Nature Power is a somewhat decent move in Gen VI. For all practical purposes its BP will never be below 80 this generation, and in grass it will even give you that sweet Grass STAB in Energy Ball. It defaults to Tri Attack in major battles, though. Uproar has also received a pleasant BP upgrade, although it arrives late-ish at level 32. At that point, you're facing Winona, who isn't the easiest foe for Lombre to face.
At level 36 Lombre learns the ever-useful Knock Off, at the same time as your starter reaches its final evolution and you might have picked up a Water Stone or two. Now you're faced with a bit of a conundrum: Do you evolve Lombre? Ludicolo is a nice 'mon, but its strongest move can't be learned after evolution. Sure, it can patch up that moveset nicely with TMs, but Hydro Pump is locked off. Fear of missing out may lead you to stick with Lombre until Hydro Pump is learned: If so, have fun with the 340 BST 'mon until level 44. That's right before Wallace.
So yeah, Lombre has it better in Gen VI, but it's still weak with poor STAB for far too long. Its coverage options have been significantly upgraded, but it still can't put that much power behind them. It's still mediocre even with the power upgrade, especially considering the relative power of the other Pokémon in the game. You're lugging around Mega Evolutions now, remember.
As for Seedot, well ... remember that bit you said about needing Bullet Seed to deal damage at all? And that bit I said about Bullet Seed not being a TM anymore? Yeah, this thing is not going to do you any favours.
Well, at least it gets the awesomely upgraded Nature Power! That's sure to be usefu- ... wait. Seedot learns Nature Power at level 15, that is, by delaying evolution for one level. However, Nuzleaf learns Razor Leaf at level 14. You have to choose between STAB and useful coverage. And don't you think I'm not meaning that literally, because those are your only two options for dealing damage at this point. Your next damage-dealing move is Razor Wind (lol) at level 20.
Fortunately, things pick up for Nuzleaf from that point on. Feint Attack at level 24 gives reliable STAB, and it's not half bad at 60 BP off 70 Attack (not good either, but serviceable). Then at 28 there's Leaf Blade, which is sweet STAB with really nice power. Your conundrum upon evolution is between Nuzleaf's Extrasensory at level 36, or Shiftry's Hurricane at 32. Shiftry has a slight edge as a physical attacker, so it might be a moot point anyway.
Anyway, I think neither of these Pokémon received the upgrades they needed in ORAS. Lotad retains its problem with no STAB until the lategame, while Seedot grows even slower than it used to although it is redeemed later on. As "end products" they are much stronger than they used to, but the journey there is still awfully bumpy for both of them.
RSE Kirlia gets Calm Mind at Lv. 21. Kirlia phase is a non-factor.
Slow Exp. Group is arguably its biggest problem.
Kirlia has serious in-game issues in later generations. I tried using it in XY, and most matches with it followed a similar pattern:
> Kirlia is underleveled compared to the rest of the team, so it is put in the first slot.
> Kirlia faces an opponent.
> Kirlia uses a move that doesn't quite KO the opponent.
> Kirlia takes an attack from the opponent.
> Kirlia has the defenses of wet tissue paper and faints.
> Kirlia gets no XP. Some other team member gets the XP instead.
> Kirlia is even more underleveled compared to the rest of the team.
But yeah, Slow Exp. group hurts it too. If you keep it in the back of your team with Exp. Share on, it doesn't keep up with the rest of the team either. If you put it in the first slot and then switch it out first turn, you keep it alive and up to speed level-wise, but then your other teammates are guaranteed to take a lot more hits than they otherwise would. It really is a millstone until it evolves and can fend for itself.