Does it support seamless roaming of clients between group of APs?

Last time I've tried, it was not supported by any open source solution.

Seamless WiFi roaming is mostly a client decision. The best you can do on AP is to:

a) optimize signal strength for coverage (stronger signals aren't always better in multi-AP deployment);

b) provide hints via 802.11k/v/r to help clients make, hopefully, better decisions;

c) forcefully drop and disassociate clients when signal is weak enough.

But if the client has bad WiFi implementation, there's nothing much you could do.

OpenWRT currently supports 802.11k/v/r, but optimizing coverage by adjusting signal strength and channels is left for experienced users to deal with manually. There is the are where some commercial offerings will do, but the result greatly varies. AFAIK there's no ideal system anyway coz physics is hard.

Well AFAIK the core seamless roaming in Unifi is using hostapd, which is the same AP software you use on OpenWrt. See 802.11r Fast Transition.

I think it should even be possible to get seamless roaming between Unifi and OpenWrt with correct configuration of hostapd.