There are two types of electric underfloor heating, the raychem type is self regulating and does not require a sensor in the floor, the cable resistance increases with temperature and it is two parallel conductors with heating chemical between them so it can vary along its length, so any items placed on the floor will cause a reduced power in that area.
However most need a sensor in a pocket in the floor to limit temperature to 27°C which is the reason why under floor heating is not that successful. The other reason is speed, I know with the small one we had in my mothers wet room looking at 1½ hours to get up to temperature and around 5 hours to get room to temperature, as a result they are no good for geofencing, they take too long to heat up.
In the main UFH is used with some other form as well, so it maintains the temperature rather than get the room to that temperature to start with, but clearly holiday homes and the like can benefit from internet control of heating, but it only needs on/off it does not need temperature control. The Sonoff and like can switch the heating on/off very cheaply, there is no need to alter the temperature from 12°C to 20°C you have a frost stat set at 12°C and room stat set at 20°C and you switch on room stat when house is to be used.
Most wall thermostats are limited to 5 amp, where electric under floor heating often needs 16 amp, so to use them needs relays to take the higher currents.
The under floor sensor on my mothers UFH failed and in a wet room not needed 24/7 so we did use it without the sensor, however if we did forget the floor would hit around 30°C if it didn't then clearly the installation was not correct to start with 27°C floor temperature with a room temperature of 20°C only gives a 7°C differential, where with radiators they are 40°C plus so 20°C differential at least, even when the wet room floor was tiled, the floor under that was plywood and polystyrene insulation and that would actually get hotter than the tiles, so there was a danger of damage to the floor under the heating mat without having the sensor working. We would switch it off once floor was dry, it was useless at heating room, at best it could maintain the room temperature, the towel rail heated the room.
There is a big difference between heating a room from cold, to maintaining the room at a set temperature, once the room is warm the heating will switch on/off, so the floor temperature will be unlikely to exceed 27°C, but to reheat the room from cold, it could be on continuous for 8 hours or more, so likely the floor will exceed 27°C making it too hot to walk on and damaging the insulation under the floor.
To be frank most of the internet controlled thermostats do not really need the ability to alter temperature over the internet, in the main we have three temperatures, 1) frost protection. 2) Eco. 3) Comfort. I have Eco set to 17°C and Comfort at 20°C and in each room there is a button on the thermostatic radiator valve (TRV) and a single press will swap Eco to Comfort, there is a dial where I can set other temperatures, but below 17°C it takes too long to reheat room so very rarely do I use other than the two temperatures.
So in real terms the only reason for the phone app to display the temperature is to tell me when the room is too hot in summer so I can manually switch on the AC from the phone before returning home, I am sure I could set it up with IFTTT to automatic switch on the AC, but the AC is too small, and it takes an age to cool the house, so I turn it on well before setting off home, the same in reverse with your underfloor heating, since it will take 8 hours to reheat room, you can forget geofencing, all you want is to switch it on the day before your due to use house, so a Sonoff is ample, only worry is over heating floor and you can't stop that without a sensor.