I tried updating my XPS 7390 last night. The process is not ideal and I ended up reverting to 18.04 (for now). The primary issues were:
- Wifi driver isn't present so you have no internet. You can overcome this by downloading the driver and installing it manually. I used the instructions here. 
- The display looks like it needs a driver because the screen quality wasn't as good. I gave up while investigating so I didn't full track that down.  
- The Dell apt repos won't work any more. I assumed (maybe incorrectly) that because of that that all this custom stuff will now have to be manually updated for a while. I'd love to know if I'm wrong there and it can just be re-enabled. 
- After all that I got worried that I'd have to fish to see if other bits and pieces were missing and, as this is my daily driver, I was worried something wouldn't work at a critical moment so I reverted.  
I really hope someone can publish a full accounting of what's missing and how to do it properly one day (maybe Dell). I'd love to but I just don't know exactly what the steps are.
If you update, let us know somehow!