[ifwinterco]

The issue that can happen is when one direction has significantly more flow than the other - in this case you end up needing traffic lights otherwise nobody can ever pull out from the minor direction at rush hour.

The other issue in the UK is massive signalised roundabouts used for junctions where traffic volumes clearly justify a proper grade separated junction like a stack, purely to penny pinch. South Mimms A1M/M25 junction is a good example, or the M2/M25 junction where they eventually had to put in free flowing slips eastbound to northbound because the roundabout was constantly congested.

That's not really a problem with roundabouts per se though, it's just bad design choosing an inappropriate junction design purely to avoid having to pay for bridges

[Symbiote]

Bridges take a lot of space (to make a gentle enough slope) and I think in many cases in Britain, that space isn't available.

Fixing one junction might just move the congestion to the next one, and if you continue fixing them you turn all the cities into jungles of concrete.