"Could it be that with "Friends with benefits" frequently one person feels a bit more attached than the other and feels much more of a loss when it ends?"

Definitely! And that's the risk people take.

You'll get party A who just wants fun, and party B who secretly wants a relationship.
But because they're a bit desperate, party B pretends they're totally cool about it, until they start encroaching on party A's personal space and needing constant attention (which is a more or less normal thing in love relationships as it intensifies) and becoming angry and demanding - and/or manipulative and subtly controlling - when they don't get their emotional needs fulfilled.

Big warning signal is casually introducing you to Mum and Dad, because "Well, we're passing near there anyway and I just need to pick up my Fatal Attraction eyeliner." Time to become 'just friends' again sharpish ..and to lock up your bunnies.

Or of course, party B could simply get themselves really hurt without being any danger to bunnies.