You know what might work better than mini-roundabouts? Proper roundabouts. No-one would gain anything by bumping up a kerb-height roundabout, more likely they would lose a wheel. But then you get the problem: what about the buses?
Well, in many city centres they have bollards that automatically drop away when they sense a bus. Why not have roundabouts that sink into the ground when they sense a bus coming, thus allowing them to cut across? You could even have pressure sensors that measure the weight of the vehicle waiting to cross so lorries would be OK too.
But, I suppose it gets a bit complicated if you do that, and who wants to be stuck behind the queue on the day the mechanism jams and buses have to bump the kerb?
So why not replace mini roundabouts with proper roundabouts where feasible? It wouldn't cost much more than a speed hump and would actually be of use.