My experience of Frankenstein, though (if that's what you're talking about with Shelley) was that it was almost (almost!) entirely ineffective as horror, and is held up mainly by a couple of really good chapters that basically lacked anything horrific or frightening. If I'd approached the book without preconceptions I'd not have been likely to firmly categorize it as horror, I think. It's more like a Greek tragedy with a shifted setting (which is more than hinted at in the full title) and even the other members of that category that contain monsters aren't usually considered horror. I'd probably have placed Frankenstein outside of horror, as more of a Greek-mythology throwback tale, had I lacked the existing bias of "Frankenstein is a horror-genre thing".
I think you're pretty spot on and fair to the book though. If you're reading Frankenstein and you're expecting a thrilling horror novel then you may be disappointed. If you're interested in the ethical questions raised by the creature being created, abandoned, and left to deal with it's strife over having no companion... then I think it's a pretty great read.
I kept reading him, though, and loved Washington Square. Hated Daisy Miller. The Aspern Papers was fun if not really very good. I'd started reading The Liar and it was very promising, but one of my kids ruined my copy before I finished and I haven't gone back to it yet.
TL;DR give him another try, TTotS isn't his best stuff, and his quality's kinda all over the place.