X99M Extreme4 and X99 Extreme3 not booting

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I ordered these parts for two new pc builds: Intel Core i7-5820K Haswell-E 6-Core 3.3 GHz LGA 2011-v3 140W BX80648I75820K 
G.SKILL Ripjaws V Series 32GB (2 x 16GB) 288-Pin DDR4 SDRAM DDR4 2133 (PC4 17000)
GIGABYTE GeForce GT 740 DirectX 12 GV-N740D5OC-2GI REV2 video card
EVGA 110-B2-0850-V1 80 PLUS Bronze 850 W
regular Intel 2/5" SSDs & SATA optical drives

One PC has the X99M Extreme4 (BIOS P1.80) and the other has the X99 Extreme3 (BIOS P1.40) 
(I wanted to get two of the same but the store ran out on Black Friday). The intent with 2x16GB memory sticks was to allow another 2x16 set if we needed to go to 64GB.

When I turn the builds on, the Xtreme4 shows a DrDebug code of B0 (memory problem), and the Extreme3 just toggles on and off.

Troubleshooting, I found that the Asrock website says these motherboards require RDIMM memory. Turns out the G.Skill is UDIMM. Is this a problem? (Crucial's memory finder offers UDIMM memory for these boards, and in a chat they say they successfully tested these boards with UDIMM memory.) And FWIW, the "G.Skill F4-2133C15Q-32GRR" mentioned on the Asrock X99 Extreme3 memory list shows as unbuffered on the G.skill website (http://www.gskill.com/en/product/f4-2133c15q-32grr). 
I also did a forum search and found a brief mention of some folks running another X99 version board and having problems running 2x16GB earlier this year. Is the 2x16GB the problem?

I find that non-ECC memory (for the I7 cpu's) doesn't seem to come in RDIMM. 

Can I use UDIMM on these boards, and do I have to give up on 2x16GB and go to 4x8GB?