Even if the extension is well made, they can still get overwhelmed sometimes, e.g. ublock origin on sites with very aggressive ads.

Maybe that's part of the issue, I've seen uBO say it blocked 8K+ ads on certain sites.

That being said, browsers are very complicated and the fact they all heavily use sandboxing now (as they rightfully should be), I guess I’m not surprised where they don’t function as intended in various use cases.

Yeah that's very true, Browsers these days are becoming more like virtual machines. I guess it makes sense you wouldn't give all your RAM to them just like you wouldn't give all your computer's main RAM to a Windows VM.