Space saver tyres, as their name implies, are fitted to vehicles to save weight and space. Obviously their on road performance is not equal to full size tyres but they are not as dangerous as many people claim provided they are inflated to the correct pressure, usually 60 psi or 4.2 bar , and are used carefully at speeds not exceeding 80 kph. The exact air pressure required should be stated on both the tyres sidewall and on the rim.
But the big problem is that many workshops servicing vehicles and motorists never check spare tyre pressures and this causes some serious safety issues. Space saver tyres are generally perceived to be only marginally serviceable, so a flat or low air pressure tyre is going to be very unsafe to drive on with tricky and poor road handling and liable to fail suddenly. Another issue is the damaged caused to these tyres while they are being run flat or used with low tyre pressure causing overheating and damage to the tyres construction.
Ideally any space saver tyre that has been run seriously under-inflated should be replaced.