I have studied this last year. Yes there are some decoding bugs sometimes. I have seen them in After Effects in some special case that you can avoid, and in some Blackmagic hardware. No issue in in most softwares (Quicktime, FCP, Adobe softwares, DaVinci etc.). Note : we are not sure the bugs comes from ffmpeg. We fixed a bug last year in the alpha encoding of ProRes 4444, since then we didn't find anything wrong besides that you should use -q 1 (Quality 1, with 4 you can have a little blur in subtle background patterns).
TL;DR : everything is fine but don't use it for broadcast.
TL;DR : everything is fine but don't use it for broadcast.