Vivaldi only has a small team. They said in 2016 they only needed five million users to turn a profit (They generate income based on search engine referrals).

Another downside to Vivaldi is they don’t push out many releases meaning that their security is likely lagging behind. My assumption on this is their custom UI takes a while to build after a new release.

Edge and Chrome both push out timely security updates.

EDIT: Typo.