Couple of things.
Number one, as everyone else said, that light is simply the wrong spectrum for trying to flower plants. That's for humans to see things which is much different.
Number two, I don't see a driver external to that light. That means it's probably one of those direct A/C driven LEDs which means that you lose half the efficiency on the power cycling as opposed to a direct current driven one. Also means you're getting too much heat. I would not suggest using the direct AC driven LEDs.
6500k isnt ideal to flower, while it still can grow you healthy plants, it doesnt explains the yellowing nor the unhappiness to me.
Mo is for sure right, they stay smaller under this much blue light in the spectrum but thats may not your problem here?
its hard to get a picture of what your light put out in real.
do you measured the wall draw? they draw 500W
would be good to be be able to compare the intensity to the CMH, getting the picture how intense or not intense your light is.
the UFO light do probably use a very shitty driver concept beeing not very efficient, and emitting more heat then needed
still the AC will be converted to DC by a bridge rectifier, it wont be half the efficency even driven with AC directly as the LED would just light up half of the time then, remember its a diode.
using a real driver is always better, but likely not part of the problem here.
