Turns out their eMMCs are defective. Here's what's happening:
The eMMCs ignore the first read instruction, and then work from the second one on. On Linux boot, initrd makes a read request that never returns, and hangs.
If you boot off of a mSD, it woris well. You can still mount the eMMC, but you cant boot with them.
I did receiver a report that someone found a bootable (doesn't ignore first read) eMMC. But that was 1.
(Disclosure, I opened a PayPal dispute and sent back the eMMCs of my own expense. But the never responded so I got all $293 back. This goes along with 0 communication even when I was trying to do the right thing. I'm also trying to start a hardware business, and chose this platform because of no embedded radios, etc, and 1gigE on its own SPI bus.)
Tldr. Defective hardware, don't buy.