If you have a hole that is not a usual hole shape(eg an L or U, a line shape hole if you will) these seem to have problems closing, especially so if you have a ring shape hole with outer and inner surface(within the same volume) and you want to merge/connect them as a single mesh, understandably this is something tools like fill holes and poly remove(with remesh after) seem to struggle with.
In addition to that, if I have a surface with the edge chipped of, like... ___n___ where the n is missing data, I have noticed poly remove with remesh enabled also can struggle here. How do you go about closing/extending? I have tried booleans in the past with varied success, the muscle brush has worked the best requiring some extra cleanup work after, but struggles in some situations giving failed boolean operation often.