I have not had great luck so far with 18.04 on my xps15 with graphics, particularly it having both intel and nvidia, but the os seemed sort of shoddy so far. So far kubuntu/kde refuses to load at all, just crashing, same with cinnamon/mate. The only thing that works reliably so far is ubuntu gnome shell, which I hate, but sort of stuck with, and still gets like an hour of battery life if lucky. I should take some time to try and fix it, but I'm usually not too mobile with it unless doing wireless survey work.
You might do well to try 16.04 for grins and just load a newer kernel (I did so for the tb16 dock enhancements), which worked fine under 16.04 on my xps and kde or any de. Everything worked solidly there prior, again 18.04 just seems shoddy/broken. I've considered reinstalling 16.04.
Even better, I'd love to go back and get arch on that thing... Arch has been great for me on my desktop, I don't even remember why arch was being weird on the xps15 (I think the nvme disks and boot disk kernel...).
I looked at the razor blade laptops ala alienware for pretty factor, but worried about the hardware compat like this too. Like I mentioned in another thread about the pretty asus, not all that glitters, particularly mingling linux.
-mb