To me, the rather dubious historical claims that the pratitioners of the Systema make are a big red flag. They claim that the Systema was developed in Medieval Russia, so that Mother Russia's warriors could fight off the land's many invaders.
So, what does the crushing defeat of the local Russian boyars by the Mongols in the 13th century (and the subsequent centuries-long occupation of Russia by the Golden Horde) say about the Systema's effectiveness? I guess the Mongol Occupation was a low point in the Systema's supposedly long history!
Given the above, it sounds like modern Systema advocates play upon the Western martial arts public's ignorance of history in general, and of Eastern European military history in particular.
On the practical application side of the coin, I once knife-sparred against a Systema practitioner, using 3-weapon fencing masks and those hard neoprene rubber replicas of A-F fighting knives. My own background is in Western fencing and FMA. I was able to repeatedly land shots on the guy's knife-hand and face--over and over. He had no sense of timing and distance, and seemed lost as soon as each bout began. I don't know what Systema practitioners do--i.e., I don't know if they just do drills, or if they engage in any kind of free-play--but this fellow wasn't dynamic in his movement. He looked good doing various knife disarm drills, but that was about it.
I don't mean for the above example to be representative of Systema as a whole (or even representative at all, since it's just one guy), but I thought I'd share it anyway, FWIW.
David Black Mastro
"The Japanese are the most warlike people in this part of the world. They have artillery and many arquebuses and lances. They use defensive armor for the body, made of iron, which they have owing to the subtlety of the Portuguese, who have displayed that trait to the injury of their own souls." --Gonzalo Ronquillo de Penalosa, commenting on well-equipped wako in the Philippines, 1582.