Obviously, I never said JS had trouble transitioning nor did I say it couldn't be done; only that it is a more difficult transition. And, going to the League without adequate proof of oneself can come at a cost (literally by reducing one's draft stock).
For example, what we don't know in Stewart's case is how much higher than 13th he might have been picked had he proven himself repeatably against SEC caliber defenses, as Darren McFadden did (picked 4th). Stewart had better first year numbers than DM. Who knows... he could be better than McFadden, but if he is, he's not getting paid for it. Why? Because he played college football for an obscure team in Oregon and wasn't tested well enough to encourage risk averse NFL teams to pick him earlier, while McFadden benefited from the glory and attention that comes with playing SEC football--and proving oneself against the best defenses in the country.
And the awards and recognition (fame) that DM carried from his college career forward versus what JS had is quite remarkable.
McFadden was also drafted by a brilliant NFL coach considered by many to be one of the rising superstars in the coaching profession.