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.