What happened with In Flight, I think, in the end, is the same trap nearly every author, even myself, has fallen into at times when writing a crossover.
Giving one side, or one specific character, preferential treatment compared to the others.
It's one thing to have a character that's purposely overpowered compared to others. But when you do that, it can be hard to check the urge to have them show off how awesome you see them.
Even in non-crossovers it happens. All of the "Uber Mage Shirou" or "Uber Ninja Naruto" or... Well, I can go on. But, thing is, it happens to a lot of writers out there for one reason or another tend to, even if they don't mean to, end up writing a fic that, by the end, can be kind of dull due to the lack of challenge.
I think Gabriel just fell into that trap. Sure, he's got a good foundation, but, like Hill of Swords, he follows the plot pretty tightly and doesn't do much deviation, which, I think, ultimately weakens the fic.
Honestly, I think his best fic of all time is "Finishing What You Start".