Saw a video where a guy said the flex on a long bit can make it wander off course in a tight space. He always goes with the shortest bit that will reach, even if it means more connections. That extra stiffness from the shorter length helps keep the hole straight when you're pushing through joists.
My friend Mark tried to drill through a double top plate last year without a pilot hole. His long spade bit skidded right off and gouged a nasty track down the stud. He learned the hard way that @ward.anna is right about that guide hole. Even a short bit can walk if you don't start it clean, but a pilot hole is like putting down rails for the drill. It just follows the path without any fight. Now he always starts with a tiny bit, no matter how impatient he gets.