When speaking of curbs (any bit with shanks ), one also has to decide far as discipline, and how that bit is used.
Western, a curb is a bit a horse 'graduates to, once he can be ridden on a loose rein, one handed, for increased signal and finesse. So, if discussing any bit, leverage over non leverage, ridden with contact, or, on a loose rein, as ;signal bit, has to be part of the discussion.
For instance, put a Spade bit , in the mouth of an un educated hrose,not trained to the level to be ridden in a Spade, or in the hands of someone who has no business riding with a Spade bit, and that bit could be a torture.In the hands of a person qualified to ride with a Spade,and horse that has gone through the Vaquero training program, it becomes an ultimate tool for a light 'Bridle hrose'
Even so, according to Ed Corrnel, when a horse was first up in a SpADE,and ridden out to work some cattles, many old Vaqueros had a snaffle bridle tied on the back of their saddle, in case they had to take hold of that horse, working those cattle
Any tool used incorrectly, is not going to work as it is designed to