In case of Vivaldi, it's features like vertical tabs, and extreme customizability for the built-in stuff (for tabs alone the options dialog is like 3 pages of checkboxes for all the various aspects of how they behave).
Also for those who use cloud bookmark/history/tab sync, people might just not want Google specifically to have that data; Vivaldi does its own sync.
I use both Vivaldi, Brave and Firefox, all have their own strenghts.
Brave now has built in vertical tabs as well: https://brave.com/blog/vertical-tabs/
Also for those who use cloud bookmark/history/tab sync, people might just not want Google specifically to have that data; Vivaldi does its own sync.