Interestingly enough, IGA's 3Dvanias had some good difficulty. I remember the first time I ever played Lament of Innocence the bosses were taking my ass off and handing it to me. The last battle in Curse of Darkness was difficult as Trevor. And in Judgment, I vaguely remember Shanoa kicking my ass time and again.
Not sure about anyone else, but I never considered Lament of Innocence a metroidvania since played in a style of a "hub world" rather than having to traverse a huge castle or outside areas like the other metroidvanias.
Also it did not have a level up system which further made it balance out more as far a difficulty went.
And regarding the last battle of COD, it actually wasn't that hard for me, the final form of Dracula was time consuming, but other than that easy IMO.