I believe UE4 is a very powerful engine but it has this terrible source issue, I stopped working on a few projects because of that and Fix up redirectors.mostly do not fix the issue while moving assets. Example if you did a level design and you used a marketplace projects after that you decided to organize these assets in a new one mesh folder and materials folder etc, Than you, will face a huge missing source even you followed the epic guideline. I posted much time for epic to fix this issue and they do keep doing upgrades without fixing the previous problems. ALL OTHER SOFTWARES YOU CAN MOVE ASSETS EASLY WITH AUTOMATIC SOURCE UPGRADE, EXCEPT UE4. I FEEL VERY DISAPPOINTED.