The pair of those two didn't like anything that wasn't directed by Barry Levinson (who was a fine director don't get me wrong) but Siskel & Ebert (especially Siskel) just wouldn't endorse narrative concepts that went against the moral's which they believed thier audiences expected. Ebert i believe, was way more open minded in his later years. I know 'Brainscan' wasn't a great film, but the 'Trickster' (who is nothing like Fred from 'Drop Dead Fred') ... was actually a pretty fun character. How could Siskel be so wrong?
I don't know. I think Siskel has liked a few off beat or tough films. I remember him liking The Fly and Seven, Naked, and Boxing Helena. While I like Brainscan, I do remember the critical consensus being poor at the time of it's release. I think Siskel & Ebert toed the difficult line often of what is considered a good film based on their criteria, but also what moviegoers would also appreciate (as in, if you like horror movies, you will appreciate this one).
Two old farts balking on a horror movie for teens? It was Freddy Krueger with virtual reality. Still a good movie on themes of entertainment, violence and their influence.
Not overrated. These guys were two of the best film critics in the history of journalism. I love Brainscan, but even I know that it's not by any means a great movie.