I'm sure the @Vivaldi folks are sick of hearing this feedback, but I'll say it anyway (sorry): the only thing that holds me back from adopting and enthusiastically recommending Vivaldi is that it's not 100% open source.
With Mozilla chasing the AI dragon, now would be a really great time for Vivaldi to take the plunge and go fully FOSS. I don't know that it gains much, if anything, by being just mostly FOSS.