Handoff is an overloaded term. If you advertise the same SSID on the same L2 network from multiple APs, clients should seamlessly move between those APs, subject to the client's own mechanism.
Ubiquiti has a feature called Zero-Handoff [1] which indeed requires the controller's participation. The APs all end up on the same RF channel in this arrangement. While I've not used this feature, multiple APs on the same channel tends to be a bad idea and I much prefer my setup of simply using the same SSID on multiple APs/channels.
In my experience, placing the same SSID across multiple APs/channels and letting the clients decide is a problem, because the clients often get it wrong. Granted, most of what I see this on are Android-based systems (FireTVs, android phones, etc), but I've also had the problem with Ring.com cameras, and with Amazon's Echo. I personally wish the APs would force the issue instead of relying on different client stacks.
The client is in the best position to manage the handoff because it knows which aps it can hear and is configured to connect to. I've seen some APs that will disconnect clients with low signals strength, but the AP doesn't know if the client can actually see something else. Zero handoff addresses this by having all APs listening to the same channel, with synchronized security parameters; whichever AP receives the packet gets to send replies. The down side is you end up with a larger coverage area but the same channel capacity as a single AP.
But they don't, really. I tried this with a Meraki security gateway+AP, plus a separate AP. Same subnet. Handoff between the gateway and separate AP took up to a minute.
Meraki insists you need to disable the wireless on the security gateway and ONLY use a "combined network" of APs to have the same SSID for roaming.
Granted, I know very little about how it actually works -- I think they use separate channels, unlike what folks here discuss for Ubiquiti.
I use zero handoff. proof is easy. connecting to a wifi L2 network for me takes ~ 500ms, i can unplug each station in turn and observe no loss of connectivity at all for that duration
note that unifi does not mention requiring controller running (and i do not have mine running ever and it works)
Ubiquiti has a feature called Zero-Handoff [1] which indeed requires the controller's participation. The APs all end up on the same RF channel in this arrangement. While I've not used this feature, multiple APs on the same channel tends to be a bad idea and I much prefer my setup of simply using the same SSID on multiple APs/channels.
[1]: https://help.ubnt.com/hc/en-us/articles/205144590-UniFi-What...