Mega C user here. I have never had any connectivity issues, my only issue was the length of time it took to update the ride from the GPS unit to my phone. A 3 hour ride would take approximately 1 to 2 minutes to update. Once you are aware of the time it takes to transfer, the data, you just accept it and deal with it. It is not a game breaker in my mind.
Transfering data in the other direction, you can transfer routes to the GPS either through BLE with your phone or with the GPS plugged into your PC. This transfer only took 10 to 20 seconds.
I use a 4iiii PM and a Garmin HR strap and have not had any drop outs or loss of GPS signal in the 15 or 20 rides I have used it.