Ubuntu 18.04 LTS Macbook Pro - Live Disk Boot Trouble AND regular boot trouble

I have pretty much the same issue on a mid-2010 MacBookPro 13 that single-boots Ubuntu. 17.10 worked fine. Upgraded to 18.04 by command line and now the graphical OS interface doesn't appear. I can SSH into the machine. I use the machine mostly as a file and web server, and those functionalities work correctly after the upgrade. The Nouveau video driver is in use, and text appears on the screen as described by numoonchld. I can get to text login using Ctl-Alt-F7. This is a Kubuntu installation, which uses sddm as the display manager/greeter. System log shows that sddm starts then exits. As part of diagnosing, I've installed Gnome desktop and it's gdm3 display manager/greeter, and got the essentially same result. Booting from USB device with 18.10 installation media hangs at some point with text on the screen. Because I use this machine as a server, I can live without the desktop, but I don't want to.

I had the same issue on a mid-2010 MacBook Pro 13" which was running a single boot of Ubuntu 17.10 when I upgraded to 18.04.

After the upgrade, the boot sequence with default kernel Linux 4.15.0-20-generic is always stuck on an infinite log loop.

Luckily I had two kernels and GRUB configured to let me choose at startup which one to use. So I was able to boot properly using Linux 4.13.0-39-generic. Ubuntu 18.04 works fine so far with this kernel, so this made the trick for me.