I have two Mastiff types mostly Presa and they are very steady. I have them around the public running free and one has had a fair bit of personal protection training including agitation training. I decided the agitation training was a bit much as I wasn't really using the dog in that way but it was good to see the dogs capability.
I must say the Presas have been pretty steady around people, but these dogs have been reared that way.
I think with most dogs you need to find a dog and breeder that knows their stuff, take some advice and go from there. Not all Presa's are the same.