This is something thats been bugging me a bit in hall of legends and raids using ults seems to almost be detrimental enless its cara or urestag because you lose so much time in their activation i probably lose a quarter of a match in ult activations and honestly dont know if they are helping me, because my team freezes and dont dps for 5 seconds every ult. My suggestion is either freeze time or allow your units to keep attacking in Hall of legends and raids ONLY it would seriously break arena so just a change for hall and raids because its annoying.
It's another strategic element. You don't want to use your ults whenever they become available, that is a waste of time. You want to wait for several ultimates to use them all together. This is a basic strategy for HOL/Raids that I thought was know by everybody? It is especially used with garrick/bloodspear ultimates, you usually use these two together when they are available, plus whatever else is available at the same time, if something else is available but these are not then you don't use anything.
And enemy ultimates most definitely stop time in COF. It depends on the ultimate, some do, some don't, just like everywhere.