> What flaws exactly?

For starters, they notoriously make configuration a difficult and shifting game of whac-o-mole to do simple things such as disabling the AI that nobody asked for:

https://news.ycombinator.com/item?id=45926779

It's also become more unstable in recent years, and I'm not even talking about 1000 tab sessions. It will crash with only a handful of tabs fairly regularly on linux.

For more flaws, look at everything LibreWolf, Waterfox, etc do to make up for them.