You should be happy to hear that while AMD still has a proprietary driver for some outlier workstation cases, the officially recommended "default" driver is the fully open-source stack.
In my experience, dual graphics on Linux is more of a pain than either solely Nvidia or solely integrated. Both my current and past laptop have had dual integrated/Nvidia setups (the first because I didn't know any better, and the latter because I mistakenly thought it was discrete only), and both times my solution has been to just not set up the discrete card at all because I've been unable to get the dual setup working in a reasonable amount of time and I don't do anything graphics-intensive enough for it to matter.
Solus is, as far as I know, working on a software solution to make dual GPU systems better supported.
it's not just Solus. they are riding on other peoples work too. GLVND (GL Vendor Neutral Dispatch) was worked on by Redhat folks.
Intel driver support is pretty bad. If you aren't running with i915 at close to head there are easy to hit graphics bugs. Running most Steam games on Linux yields much worse performance and stability than running the same games on the same hardware with Windows.
It's far from being a single variable here. WHen you talk about games, it's not just about the drivers, it's about what graphics API used as well, and how well the engine is optimized to run on OpenGL in the first place. Most engines are made with DirectX in mind so any quick port to OpenGL will lead to worse performance regardless of driver quality.
To be honest, I can put up with the less than graphics. What really got me was the regular hangs that would happen. I spent a fairly long time trying out different things to debug it, and eventually reported the hangs to the DRM list. While they were helpful, their answer was to update my kernel. It worked, but I am not comfortable having to run at head just to play old video games.
fglrx/amdcccle and now amdgpu-pro is AMD's proprietary driver package