I have recently changed to running 2x Unifi AC Lites in the room space. I didn't want to drill and mount them on the ceiling (yes, they don't look too bad - but I would rather have things as clean as possible) although that was the backup plan.
I mounted them pointing down, using the mounting brackets into the wooden trusses that make up my roof. One is 3ft and one is 6ft above the insulation.
Signal wise I get solid 5ghz AC across the house, which is what I was after - with decent speeds as tested to home server. 2.4ghz for other devices also solid.
Temperature as mentioned is the other variable, but I've seen plenty of electronics get plenty warm before. The data sheet lists operating temp up to 70deg C - and I haven't seen that yet in the roof space.
I have a similar setup (but with 1 x UniFi AP HD) mounted in the roof (+-3 ft above insulation) towards the centre of the house (2 levels), signal is perfect even on the ground floor.