Good secondary.. I guess they're good tacklers. Can they cover receivers? No.
To me, the sign of a good secondary is how good it does on its own.
How well does Iowa's secondary do when the DLine isn't producing sacks? Not good at all.
Iowa produces NFL quality safeties even when the Iowa DB has the skills to be an NFL quality cornerback. Prater, if he makes it in the NFL, will be a safety. It's the scheme.
The scheme gives lots of yards. It's one of the lowest pass defenses in the B!G.
Finally, any team who has a sophisticated passing attack dominiates Iowa's pass defense.