I developed one a few years ago, TDD and all. Automating browser testing was surprisingly difficult, and required working around some gaff to get it going at all. Nobody else seemed to be doing it, either, which was surprising. Last I checked it didn't seem any better, so I'm not too keen to do any more. At least the extension has continued working for many years with minimal upkeep.
I haven’t looked in years but last time I checked there was a real lack of any well-documented guides of how to get things going. I managed just by trial and error but I would hardly call it fun, it left me wondering how anyone makes any complex extensions without going insane.
Maybe there’s a missing manual somewhere that I just couldn’t find, but it all just seemed pretty annoying/painful. Tooling and debugging experience was really lacking.
EDIT: This is specifically regarding chrome extensions.
