As I think we have discussed before, it had difficulties at least as recently as 4.18 or 4.19 when you added or removed variables/members into classes or structs other than at the end, which is not totally unexpected. It does mean you have to restart the UE4 editor which can be slow. Also compiling UE4 C++ code in general can be extremely slow. Is is extremely tempting sometimes to prototype some new feature in Blueprint because of this, as a result you can end up with a lot of unwieldly Blueprint code that you never rewrite in C++.