The ATD was fixed to the wall with the Apple vesa mount and lg 34 was on a desk underneath it. My rMPB/irisPro was fine, in clamshell mode or with 3 displays, I just notice an increase in cpu temp to 64ºc when using 3 displays(rMBP included).
I just bought the non 4k lg 34 because it had a huge price cut, was an open box purchase and I didn't notice any ips glow, volume control via apple keyboard was solved using soundFlower(when connecting the lg using a thunderbolt cable).
I am not sure but I think you can only drive one 4k at 60hz(Redwood Ridge TB 2) + more with lower resolution.