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.
You should try it for yourself! For me, the most important thing is that it is open source. It does most of the things that Winrar & 7-Zip does, if not everything. Out of the three, I liked PeaZip's UI & UX the most, it looks the best visually as well. It gets updated frequently as well (7-Zip doesn't get updated frequently). That's all from me.