I will also talk about some disappointing mons from Hoenn. I already talked about them in detail in my Emerald playthrough series thread, so I keep it short.
Ludicolo:
You would think this Pokemon is amazing. Water / Grass in the Water region of Hoenn? Sounds great. So where is the Water Stone to get Ludicolo? Oh, in locations that you only can get to after defeating the 7th gym? And Lombre is hot garbage for 80% of the game? A Water type that loses to the Fire gym and Ludicolo is the only Grass type that doesn't want to use Sunny Day + Solar Beam. So 8 PP Giga Drain it is? Nice, great, just fantastic. God I hate this Pokemon. WHY?
Lanturn:
Another disappointing Water typ with amazing typing for the region and also a great movepool. But it comes insanely late and its stats are just unexceptable for a late game Pokemon. Even its high base power STAB attacks just bounce of even super effective targets. It is so unfortunate. If it came earlier, then EVs could at least patch up its weak stats somewhat. Sadly, in Emerald it just doesn't do very much anymore, even against Pokemon you would think it would fare better like Milotic.
Flygon:
This investment Pokemon is a scam. Trappinch has serious issues because its only usable Ground STAB before evolving is the unusable Sand Tomb. So you have to invest your valuable Dig TM. And aside from its good attack, its other stats are awful. It is the slowest Pokemon in the game. Drag it along for 15 levels and you get Vibrava, which is somehow worse?? It LOSES attack on evolution. It gains average speed but its other stats are still terrible. Good luck with that weak Dragon Breath that you got upon evolution coming from 50 base special attack. This thing can't do anything. Pump it full with more EXP until level 45! until you finally get Flygon. Who is really underwhelming for the effort required to get it. No Earthquake via level up, so you need to spend that important TM as well, along with maybe two other relevant TMs in Dragon Claw and Fire Blast / Flamethrower. And then you have a final evolution that has the same base attack stat as its first stage evolution. Then you can dish out those 100 base attack STAB Earthquakes that you could have clicked with Sandslash who has the same attack stat about 20 levels earlier and can be caught in the same location. But watch out! Flygon can also throw out some non-STAB special attacks from 80 base special attack. Uahh, scary! Yeah, this thing just so underdelivers for the amount of stuff you have to invest into it. Also a pain to train until Flygon.
Shedinja:
I will never understand how Nuzlockers praise this thing to high heavens when it is literal deadweight for most of the game. The one or two tough trainer Pokemon it can beat 1v1 are not worth it compared to its uselessness everywhere else. This thing just explodes from everything and even when it walls the enemy Pokemon, it still isn't that good because its speed sucks and its attack is only okay. You will have to use Swords Dance to get threatening but will have to run at the first sign of any Pokemon coming in that might have a single attack that can hit it, including status moves like Confuse Ray or Leech Seed. Not to mention what a pain it is to getting it in the first place. Nincada is a heaver exp investment than Magikarp and about as useless before it evolves. For Shedinja to get Swords Dance you even have to delay its evolution by 5 levels. Ugh. The old statement just still holds true. Being invincible alone doesn't win battles. And Shedinja is not even very good at being unkillable.
There are others in Emerald that are also bad but with them you kind of expect it (you know mostly because of bad stats) but these above just trick you into thinking they can actually contribute in a meaningful way.
I don’t think every type needs a good offensive move with no drawbacks or there’s nothing that really differentiates types anymore.
I don't know about that one.
Not every Pokemon needs to have access to an Earthquake clone but I think a 70 base power STAB move should ideally be available to prettey much any Pokemon, no matter what type or if it is an offensive or defensive Pokemon. One of the main problems of Pokemon that end up as being one of the worst available options, is that they simply can't deal damage in a reasonable manner. Even if the Pokemon is defensive, it would still like to have a decent STAB move to help its status moves chip down the enemy faster. I always think it is an embarrassment when you have to resort to generic non-STAB staple moves with TMs instead of STAB moves. Not for coverage but because the Pokemon just doesn't have any stronger option for dealing damage.
I don't think the problem with types becoming too similar in play patterns really exists as today we pretty much have the situation where most Pokemon have solid STAB moves available and none of that really manifested itself.
Keep in mind that I'm just going off paper here, since I've never played Gen 5, but even without Glare and Contrary, you've still got great speed and dual screens, with options for Leech Seed, Coil, and Growth on the side. Seems like pretty good synergy with Work Up Simipour (or Swords Dance Excadrill if you wanna be optimal I guess)
The problem with Serperior is that its offensive movepool is insanely barren. You obviously have Leaf Blade but then you have Return, and that is pretty much it. Coil and its high speed save it a lot but it often needs to set up a ton to do anything since teams are less of a joke in gen 5. Leech Seed is its only good utility move too. The rest is just okay. You really wish it had some kind of status move.