Had a Cap-Chur gun in the practice I worked in, right out of vet school. Used it at least twice a week - way too many of our clients had virtually nothing in the way of working facilities - no corral, no alley, no chute or headgate; I often wondered how they ever got 'em up and off to the salebarn. Hated the d@mned thing. Swore that if I had my own practice, I would never have one of the things.
You could shoot two 1000 lb cows with the same dose - one might be out like a light in 5 minutes and be down for HOURS, the other one MIGHT slow down enough in 20 minutes that you could slip up and pitch a rope on her - and she'd be right up and ready to run or fight. Too much variation in how individual animals respond, too many cases of the dart not delivering the full dose, etc., etc., the list goes on.
Tranquilizer dart gun may have it's place, but it's NOT for routine use on cattle that you intend to keep around. Most folks use Rompun(xylazine) on cattle - when it works, they're down for a while; even if you 'reverse' it, it's not like they just jump to their feet and are immediately back to normal. It's not 'stress-free', and I've had more than one die as a result of being tranq'ed with a dart gun - and I was a trained, experienced 'professional'.