MRU drives me batty. It mostly makes sense for applications (Alt+Tab et al.) but for documents like webpages the "index" is right there in my tab bar. Why expect me to remember the order I last looked at them?
On a related note, Shift+Cmd+[] doesn't work in Vivaldi for tab switching, unlike every other browser on OS X. Dealbreaker for me.
Have you tried to configure it manually? There's an option for previous/next tab shortcut in options menu, but not all key combinations seem to work.
As for your first remark, why not both? MRU is most useful for cycling between two (or similarly low number) tabs. It's not possible with "go right" function. You have (and "always" had) other shortcuts for spatial switching, which makes sense in some situations. So you can (and do) use those. Why remove functionality for key combo that works like that in other environments? Also, you have the "index" right there in your OS as well.
On a related note, Shift+Cmd+[] doesn't work in Vivaldi for tab switching, unlike every other browser on OS X. Dealbreaker for me.