Right now, nightly contains updates that are specific to energy usage on MacOS (in a good way).
Those updates are also in beta, FWIW.
I've been using it for a few years and it has been rock solid for me.
I have been using it for a few "years" now. It has exceeded my expectation for stability from a "nightly" product. I don't even remember having problems with it in last few years, and it's my primary browser.