1. Vivaldi is slow. It's so slow that you could open 2 tabs on Brave/Chrome before it opens.
2. Side loading speed, memory and CPU usage is a lot more compared to other chromium based browsers. Don't know why.
3. And UI. There are so many options in settings. So many options and yet very few of them are something I'd want to change or adjust. I can't skin Vivaldi like Firefox. There are a few toggles and sliders but their overall effect to the UI is minimal.
4. Not to mention that the UI is in no way fancy or unique. But that's just personal preference. I like Chrome's simplicity. It gets out of the way. Vivaldi makes sure you notice the chrome.
5. And then the mailing client. I was really excited to try it out so I logged in via Gmail. As email accounts are these days, it contained thousands of emails. The surprising thing was Vivaldi started syncing all of them. Literally downloading thousands of emails. For no reason other than offline search? Is that really a good enough reason to make the user wait centuries while you index all their emails? Maybe they have added some option to specify which folders you want synced.
6. Aside from that, it was really buggy. Opening emails, composing, etc. Now that it's out of beta I hope those are fixed.
I went to Vivaldi for the UI. I had heard it was customizable. I left Vivaldi while it was still loading...