Winrar can create .rar files, 7z can't due to licence restrictions. Plus, winrar has a way prettier ui, but this is not something the advanced user would care about.
Winrar can be used without a licence , it is not so difficult to have one but not allowed here. Use Google
I wouldn't say that I can't stand it (the UI of 7-zip), but I do prefer the UI of WinRar. If WinRar didn't exist, then 7-zip would be more than good enough.
I've never noticed that error because I never work in that way, not in WinRar or any other gui. I ctrl-click or shift-click to select multiple files, just the way I learned many years ago. If I did work multiple selections in that manner, then that error would be a biggie.