Just to let you know the E60 does not have brake by wire. Thats only Mercedes Benz.

Your X5 should have drive-by-wire though, identical to the E60.

Last thing, when you say build quality I think of the interior panels (creaking, stuff of that sort). You seem to have simply electrical problems (which is great, because it means they can be fixed with software updates versus trying to find an annoying creak).

If your still having electrical gremlins ask your dealer to do a full software update (instead of just updating the CCC). Sometimes dealers don't want to go through all the trouble and update each individual module which can lead to gremlins happening around the car...

Btw, E60.net is incredibly helpful forum.