i've been using vivaldi for past month(? i think) and it has a lot of features that are okay and useful but none seem to be really great. panels disabling for no reason sometimes. rss reader is as barebones as can be and feels more like a first attempt at building one.
having a mail client in browser is great as for the first time in my life i'm actually reading all the emails i get, but it has so many technical issues that made me actually use google's online mail interface to actually manage it at all, and the last time I tried vivaldi i didn't use it because it didn't support my uni's email for whatever reason. it still doesn't, and i remembered it when i found my own reddit post from 2 years ago (lol)
syncing works for some settings, for some it doesn't, and things like mail accounts and rss feeds don't sync at all, so i disabled that functionality altogether on my laptop. (i can imagine most of these issues would be resolved a long time ago if it was open-source (not even foss) as the community seems to be made up of 90% tech people, but they're a small company so i'm not sure if it would work out for them).
keyboard shortcuts are cool and make it easier to implement some vim-like shortcuts but there seems to be no way to disable them temporarily, so when i wanted to play a game i had to actually play it in firefox
also the spellchecker doesn't save settings per-page so i have to switch back-and-forth polish and english when i'm messaging my friends and when i'm posting online, but i'm not sure if that's not just a chrome problem
but the main thing that's constantly in the back of my head is that it's using chrome. i understand why they made that decision but i still don't feel exactly comfortable being a part of google's browser share. chrome also is a different engine (well, duh) and it does some things differently. I can't tell exactly which ones, I can't pinpoint that, but my brain of a 15-year firefox user just feels like something is off.