If we include mind control and think only about campaigns which actually last a decent time, I'd say "true", but I know it's a lie because technically even one campaign where that never happened would invalidate it.
If we only include actual in-fighting, I'd say that I've never seen a long-duration campaign where a PC didn't at least grab another PC in annoyance, but then I've run and played in a lot of very old-skool treasure-hunting-type campaigns and generally eschewed this "Band of really nice guys out to save the world from the goodness of their slightly emo hearts"-type of play that seems popular with people I've never met and only know exist due to the internet.
I don't think maturity has anything to do with it, unless we're talking about penis-size-contest-type stuff, because, er, yes, guilty as charged officer, that is a product of immaturity and usually involves intra-party conflict.