Here is a recap of why he failed to be successful as an NFL QB.
Enter Bill Belichick and the New England Patriots. It was believed around football circles that if anyone could make Tebow into a strong quarterback, it was Belichick.
The Patriots allowed him to play the position to see if he really was NFL material. To make a long story short, he failed to impress the team in training camp and was released before the season opener. Although he put up decent stats, they were below the standard for an NFL quarterback, so he was let go.
This is from the article.
So while he wasn't abysmal because he had an advantage of decent stats while he was on the Patriots, however, those decent stats were below-the-standard for an NFL QB is a flaw that can't be overlooked.
https://howtheyplay.com/team-sports/Why-Tim-Tebow-Failed-In-The-NFL