At least on the hardware that I've had, it's basically rock-solid in practice. I use it with high-refresh-rate monitors, I've tried FreeSync and that works, it works with all my displays, and recently the older of my GPUs (Radeon Pro WX 7100) finally got audio output over DisplayPort, as the newer of them (Radeon VII), though I never really had any use for that feature.

The acceleration, particularly with RadeonSI and RADV, and particularly as the RADV developers (independents, Valve, and some smaller companies I wish I remembered the names of) have been making massive improvements on the shader compiler side. RADV's own shader compiler (ACO) is noticeably better than the first-party AMD LLVM stack, and RADV is substantially faster than any of the first-party AMD Vulkan drivers for both graphics and compute workloads. I hope ACO in RadeonSI becomes a thing, I think it will be a major improvement.

Message to anyone listening from AMD: maybe look into making ACO your primary target rather than LLVM, it is clearly a better design for your GPUs, it has substantially less overhead, and there's no legal reason it can't be a part of all of your drivers.

As for kernel support, it is often same-day or at least it can access the displays on launch day, provided you have the latest stable kernel. ArchLinux is rarely that far behind a new stable kernel release, so on ArchLinux, same-day support of one form or another, and full support that day or some day soon, is the norm.