I think that, given the budgets they had, they were as good as they were gonna get. I'm guessing that he got an indie game's budget but really good staff and cut whatever corners he could to get a good game.
Honestly speaking, his games after SotN are pretty solid. Harmony looked great on old GBA's and for what he tried to do graphically, I suppose he had to sacrifice sound. Aria is really great in terms of controls and the sprites are mostly all-new, which was great. With the power and resolution of the DS (a resolution that more of less matches SotN and the console titles), he was able to save even more by bringing back the high quality sprites of Rondo and SotN... too bad the new sprites clash with the old ones. I think that's where things started to get weird.
The music for the games has always been top notch, even in Harmony (composition-wise, anyway). Michiru Yamane gets a lot of credit, but I think Soshiro Hokkai, Takashi Yoshida, Masahiko Kimura, and Yuzo Koshiro did some great additions. We will never really complain about Castlevania music from the IGA games (most of us won't anyway).
The stories are funky though. I don't know what to say about those. They seem like Japanese RPG cliches, but like with the Tales series, I don't think you can take the JAPAN out of the storylines. LoS shows a type of story not usually used in JPN games though, and it shows.
I generally like Iga.. probably a lot more so than I did when he was all "Forget about the N64 CV's" and "Legends isn't Canon" which pissed me off... but that was a long while ago. If he had worked every game possible into his Canon, perhaps people would dislike him less, and would just dislike the sprite recycling. I don't know, though.