Everyone else would have to yield only if all traffic entering from that single busy inbound road was going all the way around the roundabout and exited onto the road it entered from. Since few cars do this, the odd single car on other entrances gets a gap where they pull out, often causing the busy road to queue while they pass, which in turn lets others out, etc.

I agree though, the heavier the traffic the less they work. Especially when morons pull out onto it despite seeing the exit is jammed and without leaving room for other lanes to cross in front and behind of them. Also, in the UK, many drivers fail to signal correctly, giving less clues to other drivers as to when they can pull out, e.g. if you're not signalling left as you exit a roundabout, you're doing it wrong! :-)

Well you can have roundabouts with traffic lights. They work well in Chandigarh, India.