I'm currently in Germany roaming with my Czech Vodafone card. In some areas the 3G/LTE Vodafone.de signal is really bad, so I've thought it might be worth a try to switch to a different network since EU roaming is supposed to work anywhere for free. Unfortunately my phone was unable to register on a non-Vodafone network so I couldn't fully test the theory.
So, is EU roaming supposed to allow you to connect to any phone operator within a given country? Or can your home operator restrict your choice to a single network of their liking? The official roaming FAQ fails to mention anything relevant.