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>It's slow
While it is unfortunate that Pearl and Diamond played pretty slowly, Platinum raised the speed of the text in battles which greatly sped up battles. It also raised the speed of surfing in the overworld.
>It has too many Legendaries.
While it does have many Legendaries, "too many" is obviously subjective. Furthermore, if you only count those that can actually be caught in the games, then it only has 1 more Legendary than Hoenn, as the rest are Mythical that cannot be caught in-game. Also, some of them are not particularly strong, and are only Legendary for lore-reason; this means that they didn't have an influence on the overall meta of multiplayer. Lastly, having more Legendary means more content in the game.
>Mt. Coronet is a pain to go through.
Mandatory random encounters serve to give you more EXP and also lengthen the game as you go through battle. Some may think this is "padding" the game, but this is really how battles work in any game. More battles = longer game time. As seen in SWSH, being able to easily avoid battles everywhere, even in caves, greatly reduce the overall game length. Furthermore, mandatory random encounters present a challenge in slowly depleting you of your resources (HP, PP, Pokémon alive, items) as you try to go through the cave. This serves as a form of resource management challenge.
>Too many HMs
This is again subjective, as some people enjoy having HMs; it gives you a better sense of adventures, is a neat way of hiding secrets in areas you've already visited, and challenges you to beat the game with less "useable" moves in your party.