There can be side- from running custom repos like these, like mentioned in few previous posts the testing is limited. I mainly test that the repo builds on development conf and see that at least one sample project that uses each merged tech runs and builds fine on Windows shipping conf. If you look back of the comments of thread lately, few users have had different issues, one noticed that 4.20 repo doesn’t build on Debug conf, which I never tested myself so that slipped through. One user noticed WaveWorks giving a cost even unused on 4.21 so I just separated it into different branch now but I don’t have time to systematically analyze the these techs have for the stock ue4.
In general the negatives should be minimal but things like mentioned can slip through and I give no guarantees these things work. I do try to fix all known build issues if I can but that’s also limited by my time to put into (I haven’t had time to check the 4.20 FleX on debug conf for example. It must be a trivial thing to solve unless it’s something Nvidia didn’t ship required binaries for from their side).