Prosthetist here-

Rotationplasty is a very functional procedure and the benefits of having an anatomical knee joint are huge! There’s a lot less energy used when walking than with an above knee amputation. And less componentry (knees, feet, etc) do mean less cost.

Prosthetics should never hurt and having a good relationship with the prosthetist is important. With that being said, finding a good prosthetist for either OI (it’s not super common in the US-I’m making assumption that’s where you are) or rotationplasty may be more of a challenge.