This.
If the OP of this thread would take a few minutes and read through Ubiquiti forums they'd know that there have been 10's of conversations wherein someone from Ubiquiti or a power use has explained why you shouldn't be using the old ZHO hacks. I said this yesterday in another Ubiquiti focused comment, but 1) ZHO forces all AP into the same L2 network and 2) ZHO puts all AP on the same channel and SSID. The last one should be blatant enough for anyone to know that that's not going to scale or be performant. ZHO was implemented for old, low bandwidth devices that needed to be on the same network. Full stop. Don't use ZHO!
Beyond that do some due diligence when setting up your networks. Even with 802.11n/ac in the 5GHz range you should be aware of best spectrum usage, about things like DFS and consider how wide of a band you're allocating to the channel selection your making (hint: it impacts overlap). Do not do not do not use auto channel selection unless you're forced to. I get it, you may not have control over this in a dense environment (apartment) but if you can control your airspace (reasonably so) map out channel utilization especially if you have more than one AP. Then go to the next step and figure out the worse spots for reception in your house are and, as the person above me states, tune RSSI down to get clients to get kicked as they "roam". This will force them to reselect much faster. One comment stated 8 second reassociations. That's just bad tuning and clear ignorance to the problem - you'd have the same thing with any other vendor.
This wasn't hard to find and there are many others, use it: http://securityuncorked.com/2013/11/the-best-damn-802-11ac-c...
I think Ubiquiti makes some really great products for the price. Have they violated GPL? Quite possibly but as others have said I don't think that's been proven judicially as of yet. You will get one of the best products at this price point though and, when setup correctly, will be much more performance than things like Eero or Google Wifi that relies on wireless as the backhaul between APs. If you're saying you can't use cabled infrastructure there are options. 1) Powerline is now "decent" as in 400-700 Mbit are attainable depending on your situation, they're also much cheaper (look for HomePlug AV2 standardization) now 2) Dedicated, directional focused wireless backhaul is the better option than an omni backhaul between something that is already handling your clients. That is Ubiquiti's forte. They only recently released the UniFi product but you can use NanoBeam, Bullet or NanoStation in your home to move things around to dedicated AP.
Ultimately if you want good wifi you have to pay for it and plan and deploy it correctly. The second half of it is why we have Eero and Google Wifi - because people don't do that. They buy Ubiquiti or some other enterprise focused WLAN solution, plug it in and do the minimal work and say it doesn't work or sucks without RTFM.
Finally if you want "fast roaming" you should know what you want before buying a product since the device OS needs to support it. Apple has basically said they're supporting 802.11r: https://support.apple.com/en-us/HT202628. And 802.11r is in the latest builds of UniFi firmware, but not really exposed to the GUI config because, well, people will probably shoot themselves in the foot with it.
Edit: added link to channel allocation graphics