It's perfectly safe if you understand its use.
For instance, if the pressure is low and you decide to use FL70 against an inbound at Fl80 that's perfectly safe providing you don't then stick an a/c at 6000ft underneath the one at FL70!!
Separating a/c is what we're paid to do at the end of the day it's all about flexibility. As Scuzi has said, it keeps things moving
