I've seen him train kids and seen the results, even after just a couple of sessions. He would occasionally come in and do some technical training sessions with my son's team. Even though my kid is a GK, the sessions were very positive for him as well and helped him with both his footwork with the ball in the back as well as his distribution. I mean, just a session or 2 and you could see the instant difference JM was making.
My son did a couple of sessions out at Samba too with a GK specific trainer so I can't really speak to Felipe's effectiveness. Everything is different when you're talking GKs and who they train with and what they work on. Lewis Amos was a great GK coach for my son...he was crazy and challenged my son to be better and put him in a training group with GKs who were slightly better than him so that also pushed him. BUT, the best coach my son had was Donovan Ricketts mostly because he challenged my son to play the game from the GK spot in a different way and he forced him to focus on skills that weren't associated with shot stopping. He treated the GK position like it was the grand master of chess on the soccer field which was perfect for my son because he is a very analytical person.
JM's personal issues can't be ignored so I questioned the wisdom of putting him with extremely young kids but as a coach for kids 14+, he makes them better players.