@dimka The answer is simple, LineageOS is a modded Android, Ubuntu Touch is totally different: Different kernel config, different use of partitions, different structure of the initramfs etc.

So, that all makes it hard to cover every case that users might have depending on:

if they repartitioned their device if there is still data in the system partition which will prevent correct boot if data has been encrypted if the vendor partition is from a different Android release

And more ^^