Well, you have to have an outlet for oxygen to escape, along with solvent vapors. Net total pressure in your flask stays more or less constant, at least until you remove the outlet.
Using a Schlenk line isn't that difficult, but there is some technique involved. There are really only two important rules:
(A) Never pull air into your cold trap when it's submerged in liquid nitrogen.
(B) Always wear safety goggles.
The biggest hazard with a Schlenk line is explosion or implosion. I've experienced the latter firsthand, during a fpt cycle, and it's not particularly fun. This usually is only a real concern if you're pulling a vacuum.