+1, I bought the game because of it's Linux support a whole while ago and closed source drivers are no option due to their quality on the long run and as Shmerl already pointed out, AMD will only keep AMDGPU-PRO for workstations as soon as the MESA Vulkan implementation is mature enough. RadeonSI is already performing significantly better than the proprietary drivers in nearly all OpenGL games.
Valve has people working on MESA and might get rid of AMDGPU-PRO in SteamOS already in 2017. That way they can directly influence the drivers on their own which is impossible with closed sources.
As you can see on this list for official OpenGL 4.5 compliance there are only two features necessary, namely GL_KHR_texture_compression_astc_ldr and GL_KHR_blend_equation_advanced.https://mesamatrix.net/#Version_OpenGLES3.2-GLSLES3.2
Then everything necessary from 3.2 to 4.5 is completely implemented in AMD, Intel and Nvidia drivers of MESA for the current product lineup.
It might have looked like an easy approach just not to support other drivers as long as the message sais: "Hey, your OpenGL version is just not high enough!", but as soon as all MESA drivers expose version string 4.5 your game will be perhaps(as far as I can estimate from my whole steam list) the only game that crashes at startup with MESA drivers. It will then also be completely unnecessary for all games to override the GL version string so almost every other game will start without issues.
And finally as soon as Valve removes AMDGPU-PRO from SteamOS, without judging anything, just telling you: people might get pretty angry and Valve would also be unhappy with Divinity:OS being broken for all GPUs on their OS other than Nvidias'.
Others do also have few bugs in their games but the entire game still starts and works as far as I can tell by my list of 76 games - that's the big difference.