I was started English where we carried a crop or whip as a matter of dress. You don't show without one. You have to be prepared for conflict with both and know when and how to apply them. A horse who feels pushed can rebel violently about your decision to go forward with either tool, or they can decide to submit.
I've not had good experience with spurs myself, not that the horse overly objected to them, but that I couldn't keep them on and in the right position. They always slipped... And I think, while one can accidently apply spur in a moment of imbalance (and make the situation worse) one doesn't ever accidently apply a crop.
I usually apply whip to the shoulder or flank just behind the leg to a disobedient horse who won't go forward... you're less likely to get a buck that way, like you might if you hit the butt.
Yeah, you've got a point there about accidental application. Been there, done that. As far as the fit goes, I've always liked the English styled spurs that had one long strap that threaded through the slots on the ends of the spurs, so that you could get the spur adjusted on your heel so that it sat on the spur rest on the back of your boot and that usually keeps them from slipping too bad.
The ones I'm using right now are actually western style spurs that have the little buttons on the ends instead of the slots, so I had to modify my spur straps to have a button hole in them, which isn't nearly as flexible. Once you've got that hole there, that's pretty much where those spurs are going to go. So if you change boots and the spur rest is higher or lower, then you need a new pair of spur straps. But they have larger rowels that are set crossways instead of up and down, so I like that part better. There's always trade-offs I guess.