I've had some horrendous experiences using Google Maps for cycling in the UK too. Mostly it's very good, and it's impressive how good it is, really, but it's the times it gets it wrong that you remember.

Routing me through a park in the middle of the night, when it was closed, was one. Taking me on a detour to avoid a busy road, which turned to be on a farm track where the road surface was completely destroyed, was another.