I finished up the "postgame" of Legends: Arceus within the past week, and while I had my doubts before, having played through the battle against Volo twice now (once when helping a much more inexperienced friend of mine beat him), I think I can safely say that... yeah, this guy's honestly pretty overrated. I'll save you my rambling on how he's already based off of another fan favorite character, that being Cynthia (who I also view as very overrated), but I just don't see the appeal this guy has for everyone else.
First off, his character writing. By the end of the game, he tries to come across as another one of those "twist villains" along the lines of Lusamine, Chairman Rose, or even Kamado in the same game. Compared to them, there's really no reason to be suspicious of him for quite some time. Sure, he's really interested in Hisui's mythology, but so was Cynthia, so what's the problem? A good twist villain is one who does have some hints but keeps them relatively subtle. He also goes out of his way to call the player "one of his best customers", among other things throughout the story, even if you rarely bought anything from him... I dunno, as someone who wants to be a writer when i grow up, I feel like Volo does some things right here and there, but if it wasn't for people's die-hard Cynthia bias, the characters based off of her would be just as, if not even more forgettable and bland. At least Cogita has some intelligent lines here and there.
Next, the battle with him. I'm sorry for speaking my opinion online, sue me, but... this fight is the epitome of horrible Pokémon game design. As much as I don't care for Cynthia compared to the average simp fan, at least I can say that her battles can feel rewarding to complete. Both times getting through this fight, both attempts being much easier than the general public made it sound, I didn't feel anything after eventually winning. If anything I actually felt cheated because the fight's essentially a 6 Vs. 8 with Giratina also getting one free move upon its transformation. For an "epic, climatic final battle", the music didn't do too much for me to set the tone, either. Even if I cared for Cynthia as a Champion back in modern Sinnoh, the first phase of the song is just way too... almost silent, very "un-Pokémon-like" in my opinion. The only enjoyable part of the battle for me was trying to play the new battle system most effectively, but this can hardly be considered a positive when said battle system's been the same for the entire game up to this point, even if it's different from vanilla Pokémon.