There's no problem with Leon being depicted as strong. The problem is that they have overdone it. It's an overkill. Leon has been portrayed as so strong to the point that Ash winning would make absolutely no sense. Realistically, Ash should be obliterated by Leon within one episode with the way the show has depicted Leon.
The problem i have this this line of thinking is, we've only seen his Charizard do most of the fighting for him, and even though Rilaboom took out most of Diantha's team, it's not hard to see why considering what it's move set was.
Drum Beating
Acrobatics
Knock off
High Horsepower
with moves like that all but her goodra (out of the four it battled and defeated) had it bad, but then again the writers haven't exactly portrayed Goodra in a "good" way since back in kalos, kind of like how people feel about Dragonite's recent treatment.
Alain still beat the thing despite it being dyna boosted with his Ace in one move, which simply proves Diantha didn't have a solid way of stopping the monkey as easily.
That can happen.
I'm not gonna say Ash is going to 100% win, but he still has a chance, we still don't even know what leon's other 3 are capable of, but if they are like his mediocre dragapult, then Rilbaoom and Charzard would be the only threats to Ash.
Cause thing is, if all of his pokemon were as unstoppable as his Charizard then yeah Ash wouldn't stand a chance, but that isn't the case from how I see it.
Of course I also know it's probably pointless to debate this, because I know how some feel about his JN's team and not wanting them to succeed where their favorite's failed.