I just got this board, but i'm kinda confused.
Is it mandatory to power the 2 molex on the MOBO if you use powered risers?
You don't need to use the 4-pin molex PCI-E supplemental power when using all powered USB risers. There is only a small 3.3V draw from the PCI-E slot to the riser through the USB cable.
That's exactly what I thought, however during building one of those(see below 2 winx workers) last week I've experienced some really weird issues (don't remember if MB wouldn't POST or something) the warning was disabled on both and the only difference was that one had molex connected to the board and another one didn't, so I changed that... and the issue was gone.

P.S. Of course it could be a coincidence and I'm just being a superstitious ghm... whatever. but I'm not willing to spend time testing and disproving myself. so... as always the choice is yours:)
I'm sorry, what exactly did you change? Did you used the molex or not? When i read your writing is kinda confusing to understand.
Hi , i need help from you . I have ASrock H110 Pro BTC+ and 13 gpu (6rx580 Saphire nitro ,4x Asaus \Strix rx 580 8 gb and 3x Asusu strix rx 570 4 gb) and only 10 gpu work for me, when i plug in 11th and start rig, rig recognizes sometimes 8,sometimes 7 gpu, sometimes 11 gpu...when i try plug in all 13 gpus its same thing, rig sometimes recognizes..8 gpu, sometimes 12...and very rare (after 20 restars ) recognizes all 13 gpu. I have only 4 gb ram memory. Sorry for bad English, i hope you understund me and can help me.
One of the reasons can be that your risers are shorting. On that mobo they are very close and sometimes can touch and short each other. I use electrical tape to insulate them and have no problems.
Other reason can be a bad raiser, try to test them one by one.
And final suggestion is to test it with 8gb ram.