If you know anything about me, you know I am very biased against Penn State because they are Villanova's only loss right now, but I don't think there is any bias in saying that there was an evident drop-off in depth between Sean Clifford and Ta'Quan Roberson in PSU's first loss of the season against Iowa...
You can call me crazy and tell me I have no idea what I'm talking about but I don't think Roberson's lack of talent was the main reason for the loss against Iowa. Clifford and Roberson clearly did not go into Saturday's game with the same level of preparation. Yes, he was struggling to throw the ball and his first few drives on the field resulted in negative yardage but the entire offense around him completely broke down and looked like they had no idea what they were doing.... which is expected when they don't prioritize practicing back-up position game plans.
Clifford and Roberson don’t have the same game plan because they have different strengths. One prime example being that Roberson is quick. He's fast, he's got great potential to help Penn State's problem with their third-and-short run game but it seems James Franklin hosts practice all around Clifford's game plan. When injuries happen (because they very much do happen) they don't know how to take advantage of what Roberson has to offer. So now they've got Roberson running a game plan designed for Clifford that doesn't play to his strengths or a game plan designed for Roberson, highlighting his strengths, that they don’t practice so the offense can't complament him.
No team with a broken offense is going to be able to take on a team like Iowa that places so much confidence in their defense. Iowa's defense will suffocate you. They are really, really good and it is very difficult to find teams nowadays that have more trust in their defense than they do offense. Iowa is a prime example of a team that likes to end a game with their defense on the field. When they get to work against an offense who is unprepared situationally, they're going to see it and immediately take advantage of it.
It all comes down to practice and I highly doubt Roberson is receiving anything near the amount of time or reps Clifford has received since the start of the season. When you go against a defensive heavy team with a superhuman like Jack Campbell, you need to consider the fact that if your QB encounters this man coming in full speed, straight-line on a blitz (like he did), there is a very high chance of injury.
Now by no means am I saying this is only a Penn State problem. I think there is definitely a pretty big drop off between the starting and the back-up quarterbacks at a lot of schools, but the Penn State team we saw against Iowa is a prime example that it isn't the talent itself that failed, but how lack of coaching catered to the talent of back-up QB's can lose games. I am all for the 'if it ain't broke don't fix it' mentality, but there comes a point where teams need to start considering 'how do we fix it once its broke' before it actually breaks.