Might not be a written 'requirement' but FWIW, I can tell that we had regular crashing (5+ times per day) of the Policy Server for ~3 years. And now that is "fixed", we still have to do weekly restarts due to memory/handle leaks to avoid crashing. Same situation with SPS when it was in use...both 12.52 SP1.
Web Agents have been pretty rock solid though.