I am assuming he means "non-sprung clutch disc," but I don't think it's completely black and white. One of the reasons for the springs on a clutch disc is to dampen the power pulses from the engine firing. With a dual-mass flywheel, the springs on the flywheel itself along with the heavier mass help dampen said pulses. This why dual mass flywheels are typically paired with either solid discs or discs with smaller springs, since most of the dampening is handled by the flywheel.
On a single mass flywheel setup, the lack of mass and springs to absorb the vibration do not dampen the pulses as well (which results in drivetrain noise). This is why the "correct" pairing is generally with a disc that has pretty substantial springs to help dampen the pulses.
Regarding reliability, the more pulses that get through to your transmission, the higher likelihood of causing wear on the transmission. At the same time, the benefit of running a rigid disc tends to be higher clamping loads (with friction material also playing a part) and lower rotational inertia. So it's a bit of a balancing act.
