Almost everything about it. It's pretty unusable for me all-around and just gets in the way of almost all my engineering.
I use Ubuntu for everything, and customize almost everything.
Ubuntu VM in a Mac isn't what I want. I want full access to all capabilities of the system, and no hassles with using all my GPU, RAM, and direct access to all disks and hardware interfaces. If I'm just going to sit in Ubuntu all day with VM resources maxed out, I might as well it be the host OS not the guest OS.