My long story short, I've replaced the evo ssd with another, which behaved exactly the same. At least using storahci drivers it's stable, but C7 errors keep accruing, and writes are crippled. Interestingly, it works perfectly fine on the asmedia controller, though slower, and without full support of things like virtualization.
I've finally replaced the Samsung 860 Evo with Crucial MX500 and the magic happened! No more issues of any kind on the SB950 controller using storahci drivers. I'll next update to amd drivers, just to check the difference, but it works fine as it is.
Both samsung ssds went back to amazon with a detailed 1-star review of the drives. Shame on you Samsung engineering!!
Exactly like others said. I am running Linux as well, so it took me a while to figure out it is 860 EVO (I have two, so ruled out it is not only the first faulty, I tried to find other causes, which wasted more time). Luckily I have 970 EVO in the system and that worked OK for now. Need to order more though, didn't plan it as the main storage.
This is a big waste of time. AMD / Samsung should simply mark their products as imcompatible to save everyone's time.