Regenerating from cuts is fairly easy, because it just needs to create new cells, tissues, etc., until the opening is closed. But what about broken bones? Maybe new bone cells/tissues/etc. are created, but the bone would need to be exactly at the right position, or else the bone would be out of place and possibly turning the person unable to use its member without troubles.
Maybe the problem would also be the speed. If it could take minutes, it would be possible to immobilize the member so that the bone is regenerated correctly, but in seconds there's just not enough time for that.
So what would really happen with a broken bone in a body that regenerates itself in seconds?