But there's a much smaller attack surface and the incentives for attackers are significantly changed. Homogeneity is always more vulnerable to disaster, whether we're talking about food supply or chips.

At least having the option of another vendor as a fallback (e.g. in case there's a severe RCE vulnerability in ME/PSP) is a better alternative than having to shutter your entire business.

I would not be surprised if these management engines have a backdoor that can be invoked from a guest VM... and then an all-Intel (or all-AMD) shop has a massive problem.

This is the core question: is this isolation absolutely perfect, or can it be pierced in any way? Something on a severity level like Spectre/Meltdown - people would have laughed you off the stage half a year ago when you told 'em you could read kernel memory from Javascript without exploiting both the browser and the kernel - is IMHO certain to be present in either of the "management" solution, and I'd like to be prepared when the bomb explodes.

No isolation is perfect but and that is an important but for virtualization you have much higher control over what instruction you allow through so an attach which is specific to ME isn’t likely.

That said you can have a side channel attack that allows you to compromise the hypervisor and from it you can jump to the ME but this is a different story.

But isn't this also applicable to the other side? As in, black hats have less incentives to research vulnerabilities in less popular products (security through minority). I'm not certain how this balances out.

The aerospace industry has realized this a long time, and for some things they will have 3 different devices from 3 different vendors doing the same job