Firmware. Verify everything is the same in Settings -> About Phone (Baseband version, build number)
i tried with stock L preview and then went to stock kitkatt 4.4.4 and now im using multirom with cm11. the basebands are the same
Maybe i am ...
as for ubuntu touch 14.10
the files are not .zip and thus multirom recovery cannot see them
they are .img and .tar.gz
it says here to download the .zip files