I've been working with Unifi equipment for about 6 years now, and haven't had many heat issues. I will say that the hottest Unifi devices I've interacted with are the NanoHD's that are out now. Personally, I'm not much of a fan. I have a client who asked to install those instead of the AC Pro models I'd proposed, and they're pretty darn hot. They also appear to be causing him some roaming problems I haven't seen with other AP models.

I just checked out my own home's Unifi equipment:
USG - ever so slightly warm (barely)
8 port POE switch - what I can only describe as pleasantly warm
Cloud key - probably the warmest of the bunch but I don't have a problem with it

I haven't checked the AP (AC LR) but will tomorrow. The POE switch has two POE devices (CK and AP), and four other active ports. All of this is in a plastic structured media enclosure with vents but only passive cooling. But the only other stuff in the enclosure are the power bricks for the USG and switch, a splitter for the coax, and a small patch block for the ethernet.