I would say it is, until the Snapdragon models get a bootloader unlock. Correct me if I'm wrong, as I don't make ROMs or anything like that, but LOS or any other ROM other than stock must have a kernel compiled for that ROM, to suit your device's hardware.
It's like saying I'm currently running Resurrection Remix, but then I'm going to flash Pure Nexus, but still have it run the Resurrection Remix kernel (as in, not to flash the boot.img that comes with PN). The phone probably wouldn't pass the boot screen.
Just like on older HTC models, say the One X with S-ON, I can't just come from stock, flash LOS and not then manually flash the boot.img included in the LOS ROM zip and expect my phone to boot into LOS.
No offense intended, bit you are mistaken I think in regards to how Linux and android work I think

You don't (or really really shouldn't) need a matching Linux kernel to use any distribution of Linux out there, and Android should be included unless I'm REALLY missing something. You need a kernel that supports your hardware and runs Linux stable, the rest has the same interface no matter what

.
So, has anyone ever tried? Kernels outside android can run all sorts Linux configurations and as long as all the hardware is supported there really isn't any dependencies on specific kernel apis or anything that a version of the Android programs and other tools could easily or reasonably depend on. Now getting it to start correctly could be difficult without root filesystem control, but that is something that can be worked around (although admittedly that could easily wind up really hacky)
If it's a matter of the rom dev(s) saying "I tried this and it broke for x reason" or "stock kernel is missing module y that gets patched to the source and stock won't have it" then sure I understand. However if no one really has tried and is going off cargo cult info passed by word of mouth between people who don't really understand Linux and android then I don't buy it
To be clear: I am NOT NOT NOT talking about the amazing rom devs who spend countless hours toiling away for huge chunks of their personal time to bring our community these amazing platforms, and you obviously have to know your stuff to build one, but I need to hear it from the horse's mouth before I believe it
