I don't buy the idea that ceiling-mount APs are omnidirectional. Yeah, most models do pretty well at spreading the signal in front of them (dome side), but they are not made to serve clients behind them. Clients at exactly 90 degrees may or may not do okay. So if you can't actually put the thing on the ceiling, I'd recommend either
- put it on a high shelf, dome side down
- mount it on a wall, facing out into the room(s) to be served
I've done both things with reasonable success.
But don't take my word for it. You can find UniFi AP antenna radiation patterns
here. (I'd recommend
scrolling down to the "Model Summary Plots"; for me, at least, those convey much more info than the "Comparison Table".) Depending on which band you look at, the U6 Pro is as much as 15dB worse out the back than the front.