The Pseudo Legendaries are somewhat overrated and not well-designed, but not for the reasons you might think. It’s not necessarily because most of them are dragon type, as since using the same typing repeatedly is annoying, they’re supposed to be divine creatures, and therefore expected to be strong. No, it’s because all of them are mechanically repetitive: most of them are physical attackers, with just a Dragon Dance + Sweep (or in Garchomp’s case, Scale Shot + Sweep). Sure, there might be exceptions, like Dragonite being bulky with Roost + Multiscale, Garchomp having Spikes + Stealth Rock as entry hazards, Salamence has Intimidate, and Baxcalibur having unique bulkness with Thermal Body + Ice Body & Aurora Veil. But when you look at it, all of them are the same. Some of them could run mixed coverage like Flamethrower, but that’s not their main bread and butter, and even when they do, it’s just to catch Steel-types like Skarmory or Ferrothorn off guard.
However, having them share the same typing can be fine if they can work in different ways. For example, when you have Chesnaught and Hisui-Decidueye, you can’t reasonably compare the two as the same thing. This is because Chesnaught is more of a defensive mon, with Spikes to set up hazards, Synthesis and Leech Seed to improve its bulk, and Bulletproof to dodge bullet moves like Shadow Ball and Sludge Bomb, whereas Hisui-D is designed as an offensive mon with Triple Arrows, U-Turn and Close Combat (Chesnaught has to use Body Press to avoid defense drops). This incomparability can also be applied to Lucario and Cobalion (one’s a frail lightning bruiser and the other is a utility mon) or Haxorus & Druddigon (Glass cannon vs. utility mon) or Gen 1 Rhydon vs Golem. Even Emboar shouldn’t be compared to other fire-fighting Pokémon like Infernape, Blaziken, or even Cinderace (not a fighting type but like the rest it's also a fire physical mon), because at least GF tried to make it a bulky fire-fighting Pokémon, which was unique (even though it wasn’t successful).
Now look at Garchomp. You think it's unique because of its design, but the main problem is that it's just a better Flygon. Cause it has a higher attack stat, is slightly faster, has Spikes, and a more usable ability in Rough Skin. The only things Flygon has are Levitate, Dragon Dance (which isn’t much, given its mediocre attack), Roost, and Defog (which, even then, he lost in SV). It doesn’t help that Flygon never got a Mega Evolution, but Garchomp did. You could even argue Garchomp is a better Salamence because of its slightly faster speed, and Ground fits better with Dragon than flying due to being super effective against Steels (the only type to resist Dragon at the time).
That’s not to say that all pseudo-dragons weren’t designed carefully, because some are unique and broke the mold:
- Hydregion - Stands out by being more focused as a Dark-Type, due to its brutal and wicked nature. Whilst it's not as fast and initially powerful as the other dragons, it stands out because of its unique typing, U-turn, Taunt, and defensive uses in being immune to Pranksters, Psychic, and Ground attacks.
- Dragapult - Stands out by being incredibly fast and has a variety of moves that include Dragon Darts, U-Turn, Shadow Force, Thunder Wave, Will-O-Wisp, etc. It also differentiates itself by initially being weaker, as it has a high attack but less reliable physical moves, and a variety of special attack moves, but only a decent special attack stat.
- Kommo-o - Another mixed attacker that stands out for its mixed utility with Stealth Rock, Taunt, Drain Punch, Soundproof, and Bulletproof, and has a balanced stat spread of 110 Attack / 100 Sp. Atk / 125 Defense / 105 Sp. Def. It even has two signature moves: Clanging Scales, A powerful Dragon-type special move, and Clangorous Soul, which boosts all stats at the cost of HP.
- Even Goodra counts because at least it tries to work as a defensive Dragon special wall, with Hydration. It doesn’t get praise as much, tho, because it’s not as viable as the other dragons due to its mono typing, lack of reliable recovery, and no offensive power.
I also prefer the other non-dragon pseudo-legendary Pokémon, such as Tyranitar and Metagross. The former is unique in having Sand Stream, which makes it a good special tank despite its typing and acting as a support to Ground, Rock, and Steel types like Excadrill, whilst the latter at least had a unique typing in a solid defensive typing, as Psychic made Metagross take neutral damage from fighting.
Unfortunately, none of them that I just listed will be as popular or have a high-meta-defining history as the other dragon pseudo-legendaries, except Tyranitar.
And that trend would likely continue, considering that Dragonite got a mega evolution, whilst the other pseudo-dragons would be left in the dust. It would be one thing if Mega Dragonite acted differently from its standard form, like Venusaur, but I don’t think that’s likely.