If this is also brought into effect in the UK, I wonder if and how it could affect BT's MCT
I think it's ridiculous that an ISP can insist on you using their cheap mass produced, feature lacking, unreliable CPE.
I think giving the EU the option of using any device that has passed the OpenReach MCT is a reasonable requirement. Devices like the Asus's with their mediatek chipset can cause problems for anyone sharing your cable bundle. Specifically the ability to disable UPBO. The difference in my U1 band if I disable UPBO is huge, and probably kills my neighbours upload as we're too far from the cabinet/exchange for the U2 band to be used.
I'm unfamiliar with the German setup, but such a blanket ruling here would be a bad idea in my opinion. Allowing the EU to use any MCT passed modem sounds like a reasonable compromise.