It's a good channel. I've been watching them for a few months and they give a much more sober view on United than many of the alternatives. Social media is full of knee jerk idiots who throw out extreme opinions to get attention. These two may not be footballing geniuses, but at least they're not pretending to be something they're not.
I wouldn't focus too much on their number of subscribers. I imagine that they got lucky to convince their first interviewee. And from there, their reputation amongst ex-players just spread by word of mouth. I suppose it also helps their credibility that they've got a studio across the road from Old Trafford.