My impression of proprietary licensing of embedded code is that the obligations are entirely fulfilled in the upstream interaction. That is, when company A writes code used in the hardware made by company B and sold by company C, the agreement between A and B doesn't including any obligations to company C. And the more significant difference is that none of these agreements include any obligations to the customers of company C.

The normal comparison is between FOSS licenses and licenses directly from the copyright holder to the end user, from the point of view of the end user. In this case, it is always no harder to comply with the FOSS license. But for the case of being an intermediary of software that goes through intermediaries, compliance is often easier with a proprietary license, where the intermediary can be done with their obligations when they ship.

Of course, this makes me wonder if there might be a market for a company that will write and sell FOSS code to ODMs and act as an intermediary between the ODM and the brand-name seller. That is, when the ODM wants to release something including code from their software supplier, they tell the software supplier, who checks that the firmware image matches the source, and who takes the responsibility for distributing the firmware to the brand-name company; the brand-name company only gets the binary image (generally pre-installed on the hardware) and the offer, which they pass on to the end users; the FOSS supplier has the responsibility for fulfilling requests based on the offer. This would allow ODMs and brand-names to care only minimally about compliance: the ODM has to perform an extra release step (effectively, an off-site backup of the firmware source with validation that it really produces the firmware image, which is not a bad idea in any case), and the brand-name gets a page for their manual (with the offer). Also, the entity which is fulfilling requests for source would be the company that wrote and customized it in the first place, and this company benefits directly from doing it well (in terms of visibility to potential customers).

(In practice, it would probably be that the FOSS company sends source to the ODM, who makes further changes and sends it back; the FOSS company then makes the release binary and sends it to the ODM with an offer for source; the ODM checks that the binary is what they expect in all important ways, and puts it on their hardware; then the ODM sends out the hardware with the binary and an offer from the FOSS company for the source; at this point, the ODM has no further obligations with respect to the firmware, enabling them to put all of their resources into developing the next version.)