I'm not sure they're overrated, but I've noticed (at least on the Canucks board) that people look at the stats from SEL and assume that will automatically translate in some meaningful way. Everyone has learnt not to do that with junior hockey and to some extent the AHL, since the player might be too slow for the NHL, too small, not strong enough, or a million other things that would stop him from producing in the NHL.
Success in SEL absolutely doesn't mean that the player will be successful in NHL/AHL. Theres loads of examples of that. However theres also quite a few examples of the opposite.
It's hard to tell how a player will adapt, I mean look at Erik Karlsson the year before he crossed over to NA he had 10 points in SEL, the following season 26 points in NHL.
On the other hand Rickard Wallin was a leader and one of the best players in SEL and scored 46 points in 2008/2009, the year after that he was a joke in Toronto and scored 9 points.
Just like AHL-stats and CHL-stats you got to take SEL-stats with a grain of salt and look at the player and if he got the qualities to make the league.
However it's easy to call a player coming over from SEL to NA and not impressing at the training camp a bust. In reality it might take the player a while to get used to the different rinksize and adapt to the NA game. Take Rundblad as an example, his game is made for the big ice and it will take him a while to learn how to use his skills when there are less time, less space. It easy to say, well this kid was underwhelming when in reality his game has a good chance at improving fast when he is getting used to the NA-game.
Right now it doesn't look like Rundblad will make Ottawa from the start but you can clearly see how his game has improved with each preseason game.
This is also very obvious when it comes to goalies, it takes time to adapt.