Many people use their various Ultra models under Roku 11 with no problems, so the problems you are experiencing are not universal. Now we just have to figure out what about your installation isn't working.
Unfortunately, just saying your Ultras are "almost unuseable" doesn't give anyone much go to on to try to help. If you can provide your Ultra model number(s) (there have been at least 7 different Ultra models released since 2016) and describe in detail the problems you're having, someone may be able to suggest some things for you to try.
Roku Community Streaming Expert I am not a Roku employee, only a user like you. Please, no support questions via private message -- post them publicly to the Community where others may benefit as well. If this post solves your problem please help others find this answer by clicking "Accept as Solution.".
Streaming Stick 4K+ 3821 | Streaming Stick+ 3810 | TCL Roku TV 43S245/C107X