I'll throw out two factors, which I think are of approximately equal significance.
1) The "culture" (for wont of a better word) in 91/135/whatever-isn't-an-airline is just way more permissive. By necessity (and I mean that in both senses, they're smaller and must be more flexible in a myriad of ways, but also they don't have the same level of oversight and therefore must compete, on some level, with similar operators...even in 91, someone is watching the bottom line like a hawk).
2) The flying is *radically* more varied. In every way. Not just where you go, when you get there, etc., but also a million other little variables. Things like "how do I get fuel", "where (if not under the wing) are we going to sleep", and even things you wouldn't initially think of, like "are our phones going to work there?"
95% of my headaches as a 135 jet El Capitan were basically administrative.
Anyway, point being, of *course* 91/135 is statistically less "safe" (within the confines in which "not crashing" = safety, but nevermind), in the aggregate. Even as a *thought experiment* it would be, let alone the clown-show reality of people just messing up (self included). It will never be otherwise, because you simply can't realistically fly with the same strictures as 121 and have anyone continue to pay for the service.
That doesn't mean that it's intrinsically "unsafe" (whatever that means, wherever you set the bar). I stronkly suspect that flying around in the back of a 135/91 jet is still WAY safer than sharing the roads with people like the Toddler, who just park in the left lane and *invite* road-rage in an adolescent fit of pique.
But, anyway, these member-measuring contests are absurd. Girls, you're both pretty.