Ads can be really annoying and browser mining is quite original alternative but the problem is way how admins using it. Most of sites (if not all of them) never inform the visitor this is happening. Which is IMO quite serious issue because it increase the power consumption of PC (or w/e) and user's utility bill without his awareness ... so it's actually a theft.

Maybe I would agree if site admins would inform and ask for permission or at least let the user set how much cpu can be used. But no. It's always hidden and full throttle.