Hello sulla, I got exact the same problem as you. Have you solved it?
I'm using amdgpu-pro 17.10 driver under ubuntu 16.04.2 LTS Desktop 64bit.
My gpu is Sapphire Nitro+ 570 4G, on a Asrock B250 mainboard.
It works well when there're 4 or less gpu cards. But it stuck at booting into ubuntu if I plugged 5 or more gpu cards.
I want to try Ubuntu 16.04.2 LTS with an USB stick installation, but it also failed in booting into ubuntu when there're 5 or more gpu cards.
I finally solved this problem. The cause has none sense with driver or OS, but the motherboard BIOS setting.
You should enabled 4G decode or set TOLUD to 3.5G, which depends on the brand of your motherboard.
Take this link for further reference: http://ethosdistro.com/kb/#many-gpus