I'd agree with that, and honestly I often feel like teams just get enamored with the idea of a player having a "first round grade". Regardless of what the player shows in camp (or actually in-game), teams will have a tendency to look back and think "Well he wasn't a first round prospect for no reason" and seem to assume that there must be SOME way to get it right and tap into that potential.
In the case of the Vikings for instance - regardless of what they've actually seen out of McCarthy, they're probably telling themselves that they made this big investment in him, they had him graded as a first round player, and other teams did too...so the talent must be there, they just have to figure out how to unlock it and come good on their investment.
I remember hearing a guy on ESPN radio a few years back (I can't recall who) who said something like "NFL teams view first-round QB flops the same way a lot of women view attractive but emotionally immature/unavailable men: they all think to themselves 'I can fix him'. No matter how hard a QB flops, if they were taken in the first round, some team is gonna convince themselves that it'll be different for them"