How vivaldi isn't more popular with tech users that want to use chromium totally eludes me. The browser is super moddeable and the devs have so far been nothing but super open and correct to their community. I don't think there's been a single vivaldi "scandal" of note. It literally opera before that went down the drain, and is a better browser on top.
The whole "it's not open source" mantra has also been thoroughly addressed.
Also don't get me started on the brave love. It feels astroturfed. I do not get how you can genuinely shill that browser...
It's not hard at all. The real problem is that like 90% of hardware makers don't allow you to unlock your bootloader. And every year it becomes less and less common. Chinese phones used to be amazing, because you could just slap a good clean rom on amazing hardware. Nowadays (due to pressure by the ccp from what I gather) most of them do not allow bootloader unlocking anymore. Or they allow it only using insane requirements (100+ day active user in good standing on their Chinese support forum, only 100 unlocks/day, first come first served at 1am Beijing time, etc etc...)
And without an unlocked bootloader, you can do jack shit with your phone. Hence why pixel phones are so liked for custom roms. They unlock straight out of the box, no questions asked, no weird hoops to jump through.