Haha. Well, I used to be one of those people after the last jam i did with it. The problem with Unreal used to be that hot reloading after compiling your C++ was painfully unreliable and could cause project bricks that take hours to fix, and to not hot reload meant closing and restarting the engine for any changes to the code which could take a long time if you dont have SSD (base engine no project files used to take 10 minutes before I bought one) but that has been fixed up lately, so compiling in new changes is almost as fast for iterations as it is in Unity now. And compiling shaders is another one, the engine slows to a crawl whenever you add a mesh to compile the shaders, but it is not a big deal since I do my games in C++ not blueprint I can go be productive in code during that time.
Because it is C++ it is also super punishing if you write bad or unclean code with terrible crashes and vague errors as a consequence of the language's nature, which is not bad normally as it ensures quality code or else it just wont work, but in a jam it can be bad and take extra time to write in features. For example, the whole engine will crash over a nullpointerexception, and you'll only get an error with the memory address it was trying to read, and then it is up to you to attach the Debugger and set breakpoints where it might happen and check what is at the pointer in memory/what code is causing it.
Most of the problems with UE for jams come from past versions of the engine and last problems. Long build times used to be a thing too but now building this game on my ancient i5 6500 takes only about 15 minutes.