I don't think he was very disappointing.
I would say that he was somewhat disappointing.
His athleticism was intriguing, but he doesn't seem to have a "spark".
What I mean is he plays like he is focused on impressing. It's as if he forgot that it's a game were one team wins and one team loses. Tony Romo wasn't the most athletic QB to be sure, but in the preseason, he always played like the only important thing was winning.
During one preseason game Romo even defied Parcells instruction not to carry the ball himself in a goal line situation. He scored a TD but I'm sure Parcells had a few unkind words for him later.
Trey plays tentatively as if he's always worried about doing something wrong. He's got great speed, a strong arm, and I saw him pinpoint a few passes but he doesn't seem have a lot of touch when throwing to the outside.
He did seem to improve as the game went, despite his O-line of future practice squad players in the 2nd half.