Suicide !
Flying IMC with only one attitude source is an accident about to happen...... It's just a question of how long before a technical failure results in a loss of control.
As for flight with only the TC, I was quite surprised how well a student go mine performed when during training the vac pump failed, the controler reverted to no gyro procedures and the student piloted the aircraft to minima on a PAR.
The student was at the end of his IMC course and was an above average student but it proved that even a low time pilot can if he uses the correct technique fly safely after a major failure of the attitude system.