It's about the tech stack, IMO. Chromium is a moving target to maintain compatibility with, which is difficult for a team that doesn't have much C++ experience.

As a counter example, Brave is heavily invested in C++ and Rust, and I believe they could handle that work much better.

Sure but even Vivaldi has a few C++ folks on the dev team, how many more is it to maintain an existing API to changes in the codebase, especially when any of the hard parts of that work only need be done once among all of the forks.

Even if they don't want to handle it directly, this is the kind of thing a single sponsor can pay Igalia for, who have shown the ability to make entire new Chromium subsystems like MathML. There is no shortage of C++ browser developers in the world to do maintenance work.

Can't Chromium-based browser developers work together to fork the entire thing? Ideally becoming independent of Google altogether.