I am using Kubuntu 18.04. My problem is that while the boot takes ~30 seconds, the plymouth screen shows only for the later 2-3 seconds. Rest all the time the screen is black.
I've the same problem with my old PC and https://askubuntu.com/a/79959 solved it a bit - that is to set
FRAMEBUFFER=y in the initramfs. Things were better with Ubuntu 14.04 though.