Could somebody explain me the difference between AMD 18.30 all OPEN drivers(NOT PRO) and default ubuntu 18.04.1 drivers?
As I've understood, they are all open source, but different?
There are many Linux Users at AMD Forums. Possible one of them may be able to answer your question.
But you can try at this AMD Linux Forum. Probably have a better chance of getting an answer: https://www.phoronix.com/forums/forum/linux-graphics-x-org-drivers/amd-linux
I would like to use default drivers. But for my card rx 560 amd drivers are more stable, than default drivers.
With default drivers sometimes I have screen artefacts and glitches. But amd drivers are "slower" in 3d applications.
I have found that the default drivers in Ubuntu 16.04 LTS or latest Fedora are really good for gaming on Linux.
The only problem I had was Vulkan games like Doom would not run on the default Drivers. In that case I installed the "all open" amdgpu drivers.
I tested Steam Play Beta with the latest drfault drivers in Fedora recently.
I will point you to the link ...