I wouldn't trust that this dog is going to get the appropriate training.
I have a 5 year old mastiff cross. Way before they were fashion item (he's 5 and I'm never that far ahead of the trend)
These dogs take work, strong leaders and massive amounts of training. My dog had exceptional recall (better than any spaniel I've ever owned) he is completely non reactive to dogs/children/cats/small animals.
He loves men, he's an absolute man whore because he was raised on a male only military camp, taken to work every day with my late husband, encountered dogs, horses, cattle and sheep on hill runs. He's also a great family dog because he's been raised with a child in the house, and other children on camp. He walks great on a lead, but I still walk I'm on a halti to comply with the law of always being in control of my dog in public. I've zero concerns about my XL mastiff. I can tell him to "leave it" and he will abandon anything he is doing to comply. I can say sit, down, shake hands and he will comply without fail.
But that took years of work, and still now he needs physical exercise (but not too much he was diagnosed with a heart murmur) and lots of mental stimulation and playing. We still learn new tricks like roll over, spin, turn, because it keeps him engaged.
Mastiffs and bully breeds can be great dogs, they really can, but they need a great deal of time and work put into them to become this. Same as a doberman or rottweiler.
I still wouldn't leave any dog and a child alone unsupervised, be it mastiff, Labrador or cocker spaniel, because that's just a good rule of thumb. But I fear these XL dogs that are now a fashion statement item are going to end up in rescues, or worse fatalities to adults and children.
I feel they will be out on the dangerous dog list, but selfishly I hope mines geriatric by the time this comes to be and they just stope the breeding/sale of them rather than retrospectively apply it.