They’re often worse in heavy traffic situations. Especially if you have two entrances that feed heavy traffic into the roundabout at peak times, the one that has to yield can end up basically locked out of the flow.
Multi-lane roundabouts help when theres a clog downstream of the roundabout.
Roundabouts are also great if you have more than 4 roads meeting.
But yeah, in general roundabouts shine the most in low-traffic intersections that previously had lights. They make driving in those are so much nicer, drivers basically never have to stop.