I replaced mine with a Unifi Ubiquiti AC Lite a few days ago and I've not had a single drop-out or issue since. The issues I was seeing were definitely with the Smart Modem.
Did you keep the smart modem as a modem and Unifi as Wifi or replace the whole lot? I'm thinking of doing the same.
I was using the Smart Modem as an AP only, with a Fritzbox as the router. So now I'm still using the Fritzbox as the router, but connected to a Unifi for the wifi.