Original Sonoff firmware works over (chinese) cloud servers which have outages on a regular basis.
There is are alternative firmwares available (ESPEasy, TASMOTA) which removes the cloud server dependency and enables MQTT. There is a MQTT skill for Mycroft available.
Warning: In order to flash the alternative firmware you need to open the Sonoff device, solder cables on it and connect it with UART-USB-Adapater to your computer. This will void your warranty of course.
An alternative to Sonoff are the devices from Shelly.cloud which have cloud-free MQTT support already with the factory firmware.
Main drawback of Shelly: they are produced in Bulgaria so getting them outside Europe could be more expensive because of shipping cost. Currently they are hyped a lot and most of the Shelly products have a order backlog - you may have to wait weeks or even months until new production batches are available.