Self aware AI still has directives. This has a high chance of leading to a very bad situation for the fantasy world depending on those directives, as the AI tries to fulfill its purpose no matter what, which likely will be a misguided attempt. Even if it won't end badly, it will most definitely lead to a strange development.
Being in a human body will be a limitation, but that also depends. If the AI gets into the run of the mill uber talented isekai protagonist body, then yea good luck to that world. Tho even without a talented body AI can still achieve a lot, but it might be hard to lead to a world effing scenario with other powerful beings around to stop if it goes too far. AI will still do its best to try to work around powerful beings to fulfill its purpose.
Oh and if being in a human brain alters the fundamental algorithmic AI operation and logic, then it would just be a strange human and cease to be AI. Kinda like someone mentally challenged, who is beyond genius at certain things, but hopeless at others. Honestly, it might even simply cause total insanity. AI is not a human, so being in a human brain might end up really bad for it. There might need to be some magic involved to make it work in the first place, so AI can remain human while still operating as it should, instead of becoming a mentally challenged or crazed human.