Most have already said it. The plan to make Roman a legit n°1 guy was constantly self sabotaged by mindbogglingly bad booking. It's pretty crazy how we are in year 5 or 6 of this and they are still haven't been able to pull it off. Specially with a guy that's really good in the ring, has "the look" and actually has natural charisma. But they never play to his strengths for a sustained amount of time:
This has been a constant on his career:
-He gets naturally over when he doesn't have to do much acting nor script memorizing/reading. As
@stro said, once he gets the crowd behind him, they immediately play to his weaknesses and try to shoehorn him in a character he's not good at doing.
-He's booked in long ass feuds against people that are not very over or that don't have the talent to work a long rivalry. And the most shocking part: Roman loses a fukk ton for a "n°1 guy". So instead of him being a bad ass that rolls through "lesser" opponents, he gets stuck for months winning 3 matches and losing 1 big one (I'm making up that ratio but y'all get the point). It makes no fukking sense and goes against anything WWE has done with their favorites. My guess on this is that WWE is so afraid of fans booing him because "he always has to win" that they make him take Ls that end up meaning nothing for his opponents but end up hurting Roman.
-When he finally wins the belt, he doesn't have a legit run with it to establish him as the real deal. Now, some of this has been bad luck with injuries or illnesses, but for a guy that's so weirdly booked, he badly needs an "ace run" and he still hasn't had one in 5 years.
The Braun feud has been the exception to the rule and it's truly a shame.
So yeah, he's a failure but Roman is the last one that should be getting blamed for it.