if they were not distracted by their megalomania-gasms (or just sleeping, like plagueis), none of them would likely have been killed. remember that when vader picked up palps, he was getting zapped hard by a reverse-looping arc. had it not been for his poor interior design planning concerning a bottomless shaft, he would have killed both luke and vader easily. (unless of course vader learned to do yoda's hand absorbtion technique but i doubt it.) if he had any situational awareness of the possibility of vader's turn (which he openly suspected earlier on) he would not have turned his back on vader.
so let's recount what we know by film about every sith master:
plagueis: was far more powerful than his apprentice (as palpatine claims). yet he's dumb enough to get killed in his sleep.
palpatine: could have easily had the situation in hand, if he wasn't dumb enough to turn his back on a potential traitor that he already recognized.
snoke: was dumb enough to fall for a force parlor trick. had he minded both lightsabers, that scene would have ended very differently. but instead he had to make a big show of trying to make kylo appear a puppet and humiliate him, while leaving himself vulnerable.
so yeah, i submit that a sith-master's force-power is inversely proportional to their power in planning and decision making in regards to personal safety. they dumb.