AMD tends to drop support rather quickly. It's way off topic, but it's true.
For example, Nvidia has only stopped support for their 6 series (that's 6000 series) in 2015. They supported that for 11 years, right up to Win7.
X series had their final driver released in 2010 (Windows Vista).
HD 2/3/4 series had driver support until 2013 (Windows 8.1).
HD 5/6 series had driver support until 2015 (Windows 10).
HD 7/8 (debuted 2012) and R# series have driver support through today. GCN definitely has everything NVIDIA ever released beat in terms of driver support.
That's a logical fracture. You say dGPU drivers aren't a problem because 3rd parties can't modify the hardware. And then proceed to tells that's not true wrt IGPs. How can a 3rd party modify an IGP?
If anything, it's a dGPU designs that can be customized.
It's not OEMs modifying integrated GPUs, it's the driver that allows communication with the CPU in general. Something, somewhere, prohibits vendor agnostic drivers from successfully working; they need vendor specific drivers.
As ACE76 suggested, it could easily be power/heat related. The Vega component of these Ryzen APUs may be virtually the same as their big brethren meaning given the power and thermals, they could consume 100+ watts. Vendor specific drivers set a limit so the machine isn't damaged. Yes, you'd think that would be in firmware/BIOS and not in drivers so that probably isn't it unless there's a cost cutting component to it (would require non-volatile memory) so they handle it in driver instead.
Another thought is that all of the overclocking components of the driver are excluded by OEMs as part of their licensing/contractual agreement with AMD to prevent damaging hardware and creating warranty complaints.
TL;DR: HP says "jump" and AMD asks "how high?"
Edit: I'm getting the impression RTG doesn't even know what the problem is. At least not yet. The outcry over this has made it a priority.
Edit: It is isolated to HP Envy X360 as per the Redditor that reported it which, coincidently, was the first Ryzen Mobility product ever launched. It's a special cookie.