Ok, so I did test all my GPUs with a used mobo and my PSU and they all work, to my relief.

I RMA'd the 2 faulty mobos and I should get an H81 replacement this week. 

I'm obviously going to test the replacement without riser cables first to assess whether it works or not. 
However, if -like I'm now convinced- the riser actually fried the 2 previous mobos I'm not going to plug them in without somehow testing them beforehands.
I really don't know how, though.
I did examine them but didn't find anything fishy (apart from an apparent saving on solder).

Can I not measure voltages and such with a multimeter before actually endangering my new motherboard?
I can't find anything on the forums on that matter.

As a matter of fact I am. 
However I did double, triple check that I didn't make such a mistake. 

I have 4*2 slots labeled "6+2 PCI-E & 4+4 CPU" where I plugged the GPUs and CPU.
I connected the molex cable to one of the 3*2 slots labeled "peripheral & SATA", which is the only place where the cable provided would fit, anyway. 

I have no idea what might be wrong.
Thing is I got another mobo a couple days ago, an MSI z270a, and it can't seem to detect any GPU either (I tried with the 4 I already had and with one brand new out of the box).
Either my riser cables damaged the new motherboard (if that's even possible?) or the problem is my PSU (wouldn't know how to assess that) or I got yet another defect (not entirely implausible, I got it from a somewhat shady shop and the box wasn't even sealed anymore, I'm going back tomorrow to see what they might have to say about that).

Ok, unplugging the mouse and keyboard didn't do jack. 

I'm getting more and more convinced there's a major issue with this board.
I tried booting up smOS via the onboard VGA after reconnecting my 4 GPUs and I was greeted by the "please install the PSU's 4-pin power cables to the 4-pin connections on your motherboard" screen DESPITE using powered risers AND having those 2 molex connected to the PSU... 
I ignored and booted anyway and the whole thing restarted after 10 minutes or so after a fan gradually started doing a funny noise. 

Also smOS couldn't seem to connect to the internet. 

I'm bummed. Big time.

That's weird enough it might just work... I tried disconnecting my PS2 keyboard and USB mouse and still didn't get an POST, BUT the ethernet port was blinking differently so maybe there's something to it. Currently going through a power cycle to test that theory from a fresh state. 
Considering sacrificing a goat in a satanic ritual just to get all the odds on my side, as well.

Pretty sure I have. I'll try once more before I try to mine anyway to see if the gpus are actually usable when using the onboard VGA. 

Welp, still no POST when main video adapter set to PCI-E. 
I could probably mine anyway by using the onboard VGA but something's definitely fishy there.

ASRock support isn't of much help and vendor hasn't replied in a while.
Don't know what to do.   

Ok, I'll look into it. 
I still have the "SM bus controller" for the location PCI bus 0, device 31, function 3 that gives me a code 28 (not installed), which sounds like it could have something to do with it...

I'll try and reinstall some constructor drivers while I look into smOS and amdOC.
I actually do know a bit about smOS as a coworker of mine with the same setup has been using it as of late.
Any advice regarding installation? Should I plug everything back in before reinstalling the OS?

Thank you!

Sorry for necroposting but I'm currently going through the exact same problem.  
Same mobo, works fine with onboard VGA but won't output anything through GPU (sapphire rx470 nitro 8Gb).  

I tried fullzero's solution, doing a full power cycle and retrying with only one GPU in the x16 slot without a riser but still no luck for me.  

Is there anything else I can try before sending this back to the vendor?
Delaying some more would suck big time... 

Edit : Ok, I managed to boot with the GPU plugged in by going back and forth in the BIOS and setting the video out device to "onboard".
When I go in window's device manager, the GPU is recognized as "Microsoft Basic Display Adapter" with the status:
 "This device is not working properly because Windows cannot load the drivers required for this device "code 31) The driver trying to start is not the same as the driver for the POSTed display".
I then installed AMD's drivers and rebooted. Now I do have an output on that card, locked down to 640x480, for some reason... 
Not sure where to go from here.