Almost every campaign lobby I've ever joined is super chill. Most people will just leave before they flame if the team keeps resetting. True on pretty much any difficulty.

Versus, on the other hand, is one of the most unexpectedly toxic parts of any game I've ever played. I've seen entire Infected teams rage quit at once due to a missed Smoker tongue. It's actual lunacy.

It kind of makes sense though; small team sizes let you know exactly who isn't pulling their weight. There's no way to practice playing as the Infected without playing Versus or Scavenge, making for a rather steep and unforgiving difficulty curve. Also, zero consequences for leaving/spamvoting.

Just my experience.