UD is becoming more performance oriented than ever before, and can cross our IB to Ethernet gateways. CM cannot cross gateways which is a major issue for compatibility.
Linux still has both CM and UD modes, but no other OS that I can see has it (VMware, Windows, Xen, etc).
That being said, RSS, TSS, LSO, LRO should help alleviate UD issues from the past, so hopefully UD will be sufficient for you.