Based on my personal experience, my 12v pins in the 24pin main harness melted before the slots were damaged. This was with x3 7970s plugged into three un powered x1-x16 risers. My motherboard did not shut itself down to protect itself. This was in an open air (milk crate) mining rig when I first started. An extreme case I would think/hope.
From the PCPer video, they went into detail explaining that the pcie pins were rated for 1.1x5=5.5a total, unlike the gpu 6/+ pin connectirs that have much more to offer. Im eagerly awaiting to see AIB products.
I had a similar issue, on a 4 way classified evga x58 mobo and 4 x 480gtx - my fault was i forgot to plug in the pcie aux power plug - When i noticed something was wrong the system would shutdown like it was PSU OCP. It did this about 4 times as i thought i was just hitting the limit on PSU's. then i decided to just double check the system and found 2 wires on the ATX 24 pin melted at the ATX connector and down the modular 24pin cable. The end result was i replaced the 24pin cable, and removed melted plastic at the mobo atx plug. The mainboard still works perfectly fine to this day, and the PSU i had also still works. After that, i made sure to use the AUX pcie power on it and any Mgpu setup i have done since.
The OCN user Cubeman reported he lost audio on his Rampage IV Extreme with tri-fire rx 480 in this thread. It's interesting because i have a buddy of mine who had a similar issue with his Rampage IV Extreme and he had 4 GTX 570's in his and his sound is broken. I will reach out to him tomorrow and find out exactly what his audio issue is.