Ubuntu on New computer with MSI b450 board and RYZEN 3600 - Problems with IOMMU

I have had troubles installing Ubuntu on a new computer with an MSI B450 board and a RYZEN 5 3600.

First tried with Ubuntu 20.04 then downgraded my stick to 18.04 but still unsuccessful at first.

I finally managed to do it with 2 different SO threads :

Unable to install Ubuntu 18.04 on AMD Ryzen 2500U with Radeon Vega 8 Gfx

https://unix.stackexchange.com/questions/519758/amd-vi-completion-wait-loop-after-a-failed-install-of-arch-linux-on-a-blank-ssd

I had to add to my grub : nosplash nomodeset noacpi=off and from the latter thread iommu=off

Then I have been able to install UBUNTU 18.04 properly on my SSD.

Yet I have now to manually add iommu=off when booting.. or Ubuntu will not launch and I will get a purple blank screen.

I guess I should tweak my GRUB for good.
Yet I am not even sure what IOMMU is for ? and if iommu=off is the actual solution.. I have read Iommu was about the graphics (my graphic card is indeed an AMD Radeon 550 2GB).

I really don’t care the graphics as I bought this graphic card just to be able to connect a screen to the computer (I don’t play games). Yet I’d like to know if adding iommu=off will somehow snowball to other parts of the computer / decrease performance or create instability whatsoever .