I've been a dog owner most of my life and grew up with a German shepherd. I'm very comfortable around dogs, but even I'm extremely wary of these.
My friend lives in a maisonette and her next door neighbour has a dog that looks very much like an XL bully to me and is an un-neutered male. The fences between the gardens are low, a lot lower than waist height, and she's scared to let her small dog out into the garden or go out there herself when the neighbour's dog is in the garden. The dog could get over the fence really easily if it wanted to (even my Lakeland terriers could have got over it, tbh).
She's also scared to report it, for fear of reprisal from the owner. I've considered reporting it myself, but worry that she would get hassle from the owner anyway. She lives on a small estate where there are lots of young kids, and the maisonettes open onto a play area that's full of kids at weekends and during the holidays.
I've thought about reporting it myself, but worry that he might blame her anyway and make her life a misery. He's very volatile and verbally aggressive. It's bloody awful.