Ristolainen makes a great first impression. He's got size, a big shot, and plays with physicality, and can complement that with eye-popping wizardry in the offensive zone. He stands out, so I'm not surprised that a lot of people thought he was great. It's equally understandable that the reputation stuck around for a while, as it's always hard to admit that you were wrong. He always was a player with plenty of flaws in his game, and the way he got exposed to tough minutes in Buffalo hasn't helped. He's a tremendous PP weapon though.
The timing for this thread is odd regarding Ekblad. He had a terrific rookie year in a very sheltered role, so he got a bit overrated at that point. Since then, his usage has been pretty much the reverse, which had the expected effect. People forget that he's still young though, and this year he seem to be performing spectacularly from what I've seen.
So I'd say that what went wrong with Risto was that he was never given a good spot to properly develop. Nothing ever went wrong with Ekblad, he just needed time to learn to handle the increasingly tough usage. Ekblad is easily the better d-man right now, despite how strong Risto is on the PP.