Hello Memnarch. Thanks for your posts. Interesting reading. We are in a similar position and are a hardware manufacturer. Attempting to resolve through trial and error and appreciate your efforts and public postings.

Q: How many passthrough devices do you currently have in your box?

According to the VMWARE docs, a max of 6 are permitted per VM:

Add a PCI Device

The D3D0 are power states of the respective attached hardware. Often, to save on power, the main voltage rail is turned off on the respective PCIe function and then the widget is expected to remain active through the use of the 3v3_VAUX rail. However, most designs do not make use of that power rail. Ethernet adapters may be an exception. I would not expect USB adapters to have such support. From our review of USB host adapters, we purchased a random 5+ adapters from Amazon and we could not find a single board to offer the "mandatory" USB current protection via polymer fuse and/or USB load switch to shut down the leg of that USB connector in case of excessive current draw. Rather, the only "current limit" was the PCB trace width which will act like a fuse and is quite dangerous. Hoping your situation is different. Many motherboard based USB ports do offer some level of current protection but may be lacking on ESD protection.

From other readings, users are reporting success by using earlier versions (ie. 5.5U3 appears to be ok) of ESXi so not sure if that is an option for you.

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