I don't feel comfortable with any dog (over a certain size, it's not like something small can't be swatted away) that has a certain intensity or demeanour to its body language. Comfortable is the wrong word perhaps, but I'm wary of anything that is too forward or over bearing in its behaviour. Most of the time you can tell if a dog is just being overly friendly/playful as compared to potentially a defensive / aggressive state of mind. Some dogs do just sort of... snap though, usually out of some sort of fear response (mistreated by a crap owner at some point in its life).
Even something quite a bit smaller than an XL Bully can potentially drag a fully grown man around if it wants to and a harness as opposed to a collar or a halter can let them use their full body weight. Even a 12-13kg dog can be a proper nuisance, especially if it gets a run up on a longer lead. I still don't think XL Bullies, in general, are the problem. It's some of the owners, but at the same time I do think they're an entirely unnecessary breed created by morons. Our fault, not the breeds.
Yeah there are some dogs I am wary over, especially if I'm walking my dog.
If they look strong or aggressive I'd cross the road (as an example)
I know could save him from a Jack Russell.
But anything beefy you just don't know because, you can't. It's a strangers dog.