An electrical failure would lose your T&B, and potentially the iPad mini, depending on the state of charge at the beginning of the flight.
The radio is out too.
So, whiskey compass, altitude, and airspeed still functioning indefinitely.
As long as you are a daytime, VFR only pilot, that should be safe enough, but my first and second inadvertent flight into cloud at night were at less than 50 hours, and less than 150 hours, so I am touchy about the likelihood of that happening. Neither forecast had clouds below 12,000 feet. A fully functional 6 pack and excellent instruction prior to the events made them non events.
I agree that the iPad mini would be a fine addition to an otherwise traditional panel.
The plane that I flew had an early Garmin, coupled to the autopilot. From time to time, it stopped updating, with no alarms. ATC would call up for an off airway warning, the PIC would reset the Garmin, and everything would work again. That is not something you want to deal with on the mini, as you will be losing both the attitude AND directional gyro simultaneously, even if the electric system continues to function normally. I don't think software running on an iPad is more reliable than a Garmin panel mount. Certainly, my iPad has not been.