AvastUI performs all the "visible" (UI) operations - which the service running on a different session can't do.
So if you disable it, you won't see anything - if Avast blocks or deletes a file, you won't know (so it may get quite confusing if your files are disappearing and you don't know why). Some functionality won't be available (context menu scanning, inserting license files, ...).
Shields should continue to work even without AvastUI, but it's not really the usual test case, so I cannot rule out the possibility that some functionality will also be limited.