Typing this on my x395 (with 3700u) and can confirm these are really well made laptops.
Linux support is excellent. Openbsd is good but doesn't support this generation of wireless cards yet, which is quite inconvenient; next version perhaps.
Integrated Vega full support landed in June 2018 (kernel 4.17, Mesa 18 for 3D), as long as you have these or newer and a firmware for your card in the system — you'll get full acceleration.
I've been running Debian Testing on 2400G since 04/2018, where I had to put firmware and compile the kernel with the config changes to enable the support, but it was already in kernel tree nonetheless. It's out-of-the-box since 07/2018, way before the release happened.
At least on Arch, X doesn't even start 4.19 (linux-lts package), but works fine on 5.2+ (linux package); probably also in some kernels in between, but I only bought the laptop recently.
Likely specific support for 3700u was introduced at some point between these two kernels.
AIUI 5.4 is meant to become the next LTS, so it won't be an issue going forward.
There's the amdpro drivers, which nobody uses; they're partially open source, as I understand it.
The point is that AMD releases detailed documentation for every GPU they sell, and thus it's very easy for mesa to support their hardware; AMD themselves also contribute code to Mesa, of course. This is in contrast to NVIDIA, which even uses encryption and signatures to twart nouveau efforts. I avoid NVIDIA entirely, for this reason.
I haven't had issues so far. Life has been good, same as with the vega 64 on my workstation.
GPU Accelerated graphics (2d, 3d and video codecs) on both Linux (5.2+, probably earlier) and Openbsd (6.6+), with open source drivers (kernel DRM, userspace mesa3d and xf86-video-amdgpu).
If you get tearing, try:
xrandr --output eDP --set TearFree on
To enable the anti-tearing workarounds. This shouldn't be necessary on a modern composited desktop, but I do need it with a simpler i3 setup, to not tear videos on youtube. Mpv seems to not need it either.
Linux support is excellent. Openbsd is good but doesn't support this generation of wireless cards yet, which is quite inconvenient; next version perhaps.