Here are my reasons:
-I don't really trust a chinese rom, even if an effort has been made in the EU rom to stop using Chinese servers
-The UI is terrible in MIUI. For example the volume control is terrible in comparision to windows phone or CM. In MIUI you can't even reduce the volume on the phone without the phone making a noise! I know you can use the silent toggle but that's not what I mean.
-MIUI feels like an iOS clone
-The EU Rom community is not the most helpful of communities. There are a lot of questions in these forums and a lot of people with the knowledge to answer them but most go unanswered...