Indeed. The Lex Luthor of Smallville, by season six, could certainly qualify as scary. Far more so than Eisenberg's over-the-top portrayal. The problem is that whereas Rosenbaum's Luthor says only what's necessary and nothing more, Jesse's Luthor says a whole bunch of nothing. He always monologues and there's no real point to it...and if there is, he rambles on so long that you don't even give a damn by the time he gets to whatever point he's trying to make.
In Season 5 he was edging closer to the darker realms, but you always got the sense that everything he was doing was because he believed it was the right thing, no matter warped.
By Season 6, he didn't care about getting his hands dirty anymore, nor did he care who he hurt. His deviousness by manipulating Lana and the pregnancy was also pretty sick, and I mean completely shocking.
And by Season 7, as we know, he went full on dark side, but with that maintained justification that everything he did was for the benefit of mankind, completely overlooking his own selfish desire for ultimate control. By being the villain, he thought he could've been herald as the hero by saving the world.
Season 10, he developed clones to ensure his supposed destiny of being humankind's champion could survive.
He was a sick, twisted, evil and scary bastard, and for that reason, playing with so many different layers of the character, Rosenbaum remains the best onscreen Lex Luthor. There's no one else who can compare to that.
Eisneberg... The only thing about him that scared me was popping his cherry into the guy's mouth.