I clean flashed M8.1 with 4ext 1.0.0.6RC3 and I'm getting the 4ext safe mode, because of the cache partition. Also can't backup, because 4ext can't mount /data partition.
Other than that the rom seems to be working fine.
For me battery life is quite important. It has to be able to provide at least 6 hours of screen time.
Then the RAM has to be big enough for comfortable multitasking.
Also the phone has to be durable.
Also it can't be done due to licencing reasons. I think I once read somwhere on this forum that someone was trying to port it some phone, but Microsoft didn't like it.