I have a Roamio Plus and don't experience any bugs, crashes or sluggishness with TE4.
It's greatly improved since it came out in fall of 2017.
Many of its detractors are still soured by their initial experience 4 years ago.
It's completely usable for day to day use unless you must have the TiVo guide view or transfer shows from an external location.
Interesting. I used TE4 up until recently (about a week ago). The forced ads (which often didn't even work right) and occasionally crashes and other issues made me go back to TE3 and I am very glad I did. Oh, one super annoyance/bug was that I disabled 4K video modes on my Mini VOX, because my receiver doesn't support it, but at least two times it seemed to revert to a 4K signal, so I didn't get a picture until I had to reconnect it direct to my 4K TV and change the setting back. It was another waste of time and swapping cables is not fun with my setup. TE3 just works, like a TiVo should.
When I first "upgraded" to TE4 a few months ago I thought they must have had plenty of time to get all the kinks... but I guess not.
I'm just surprised if anyone says they didn't experience any bugs, crashes, and sluggishness because I sure did, and that was up to about 1 week ago when I made the switch back to TE3... this is all on a Roamio.