We've had really great luck with Unifi and the access points are cheap and easy to manage. Now that there are mesh systems, perhaps this will change (the Eero is supposed to be great), but for the price, nothing is as good.
However, as we added more users and intensity, suddenly the "square" Unifi AP AC has started to crash. Ashe " I've not had to turn on debugging, here are notes on things to do:

1. Get the Cloudkey, this is a $80 mini computer that runs the Unifi controller, so you can always monitor the APs remotely. At that price, it is hard to ignore 2. To do debugging, you can look at the Unifi logs and we see that the UAP AP AC are rebooting under heavy load as much as every 15 minutes. This doesn't seem like it's a problem with the round ones. I recall the square ones used different chips and never quite fit in. Beside being very hot. 3. Make sure you lower their loads, by lowering their transmit power, we moved them to "low" which seemed to help and add more AP ACs as it seemed load dependent. The access points with say less demand seem to work fine. 4. And yes we upgraded to the latest firmware and controller software. 5. In looking at this, I realized that I had not configured them well, at home, where it is relatively quiet setting them up for "automatic" and very wide channels (HT40 at 2.4GHz and VHT80 at 5GHz) seems to work great, but at work, there is lots of interference, so according to the forums, it makes sense to force sectorization and go to "low" power and narrow channels (HT20 for 2.4GHz and VHT40 for 5GHz). Then you can put them closer together. The solution seems to be to move to the "round" APs and there are couple of new choices: 1. Gen 3 or the Unifi AP HD has arrived. Although$300, these are 4x MU-MIMO and with the latest MacBook 2016, you can get 1.4Gbps out of them. To do this, you need a pair of Ethernet cables and you have to bond them, but the work.
2. Gen 2 or Unifi AP AC Pro is the direct replacement and is nice for low noise
3. Unifi AC Lite is lower power and then you can get really low cost for each room kind of Wifi, not a bad choice and very cheap. This means you have to have Ethernet in each room though.
4. Unifi AP AC LR for outdoors and big distances.

Net, net, the recommendation is to move off the square APs, they have lasted four years, so maybe that's the life, get the new circular ones and a Cloud Key for each site.