I've had a RS720Q-E8-RS12 for a couple of years running ESXi on two nodes and compute tasks on the other two. I think I was a fairly early adopter, and the install was a bit rough, with one of the node motherboards being bad and needing RMA - I had the same issues above, with ASUS even accusing me of removing the serial number from the box! No replacement for the 2 CPUs it cooked either.... The early BIOS/firmware was a bit rough (and still is - at least one of my nodes thinks both of the power supplies have failed...), and I made the mistake of using a Z10PH-D16 iKVM firmware, not the RS720Q-E8-RS12 specific one, at one point. That node still has different fields in the firmware!
What I wanted to report was that I had to replace the M.2 boot drive in one of the ESXi machines a couple of weeks ago. They have two CPUs, so I decided to try the newish Toshiba RC100 drives. These are the only M.2 2242 NVMe drives I have seen. They are "only" PCIe 3.0 x2, so they are not the fastest NVMe drives, but they are still much faster than USB... I couldn't find the 480GB versions anywhere, but the 240GB versions were only $80, and are big enough for both boot and some local flash cache on ESXi. I've only done the one node so far, but it booted up and ESXi 6.0 installed without any issues, and I've not had any issues since. Haven't fully configured the cache yet, because I haven't had a real maintenance window to do the other box in the cluster.