IK bones are helful for a lot of things, but need to be manually set up and corrected (in animations too for performance reasons - never let the engine do what could already be done by you).
So, no. There’s really nothing else that would prevent you from using your custom skeleton except for the ease of using someone else’s animations (as bad as all the free stuff is you aren’t missing out on much).
Other things you may miss out on is some hard coded stuff when it isn’t of your making - and it isn’t properly made.
Some old marketplace stuff does have things like that where the code checks for bone names and sockets things.
Neddless to say differnet bone names would throw erros for those items.
My 2c, you are almost there. Just finish the rotations and you have a viable thing ready for retargeting.
Even if you don’t use it now, if you make any custom animations you can then target them onto the epic skeleton and apply them to a mannequin or any other character that’s compatible.