This is repeated frequently, but I was never able to get my AMD 560 to work on Linux at 4k with MST (or turn down the fan speed). With Nvidia I installed the binary blob and 4k over MST “just worked”. I tried the amdgpu and Pro versions of the drivers.
Neither company is perfect with Linux support. As with most things, there are pros and cons to both sides. But for anyone else reading here, just because you pick AMD does not mean everything is magically trouble free on Linux.
Does MST even work 4k@60 over DP? 4k@60 requires so much bandwidth, that is uses both MST channels (it is internally handled as two monitors); if you want to use both channels separately, you are stuck with 4k@30 at most.
Yes, MST absolutely can do 4k at 60hz. This is what the first Dell 32" 4k used.
My rMBP has been able to do this since February, 2014. I was trying to get my RX 560 in mid 2018 to do this and it just was not possible via any means with Ubuntu 18.04. I ended up using an Nvidia 1040 with the binary drivers without issue. The nouveau driver for the 1040 was able to do it with a newer kernel than Ubuntu 18.04 had (I was using Arch for that test).
To get two 4k@60 outputs, I have to run two cables (it has two TB2 ports). With daisy-chaining, I got mirroring at best (I didn't check refresh rate at the time, though).
I've never tried MST on AMD so I cannot comment. But of course it goes without saying that by picking NVIDIA it won't be magical and trouble free for everyone.
Neither company is perfect with Linux support. As with most things, there are pros and cons to both sides. But for anyone else reading here, just because you pick AMD does not mean everything is magically trouble free on Linux.