Agreed. Actually, Jonathan Frakes is amazing in the right episodes (oh don't get me started...kind of a Trek geek here). I think the fault lies with the episode itself. I mean, the concept is definitely ahead of its time and very daring for prime-time television in '92. I just simply don't buy Riker and Soren's romance, and I think that has more to do with the way the episode was written than Frakes' acting. It feels rushed, but I guess there's not really much that can be done about that given the time constraints.
I also feel like it would have worked better with another actor, or actress even (come to think of it, a cis-female in that position of falling for the androgynous Soren like, say, Troi, would have been great and even more daring). Riker is just too much of the cliched ladies' man, and it's clear this is a "love of the week" as opposed to a real "I've fallen deeply in love" kind of thing that the show tries to convey. It doesn't work.