I don't really see why Riley "should" have been comparable to a Willow, a Xander, or a Giles.

Of course Riley didn't have as much time to develop as a character as Willow, Xander, or Giles, but like I said, there's a sort of hierarchy on BTVS. Or rather, more like a web, with Buffy at the center. Characters that branch directly from Buffy (Willow, Xander, Giles, Angel, Spike) have a tendency to be much more developed than characters that branch off from them (Tara, Oz, Anya, Jenny, Drusilla, Darla, Harmony). Riley breaks that pattern. It's not just a matter of him being boring as a person--Xander is, too, when you get down to it--but just a lack of establishment of his own personhood.

Time isn't really an excuse for that because, by the time Riley left, Spike had been a regular character for about as long as him, but was far more developed in that time. He'd gone from being the Big Bad hunting for the Gem of Amara to a suicidal Former Big Bad to a demon killing machine to a neutered puppy in love with Buffy against his own will. He'd established a pretty solid (though antagonistic) relationship with Willow, Xander, and especially Giles, had an independent relationship with Harmony, had his own motivations, and we got Fool for Love which delved into his backstory in a way it hadn't been before and gave his character more dimension. And he was just comic relief for most of that time!

Riley didn't really get any of that, though. We got some lipservice to a life in Iowa and the military before Buffy, but we never get to see it. It's generally accepted that Riley had decent relationships with the Scoobies, but that's really only ever seen when he interacts with Xander (almost exclusively about Buffy). The only way he really changes in terms of how he thinks and behaves is, as you mentioned, he becomes bitter and insecure because of Buffy. By the time Riley left, I think even Anya had established more space for herself.

/u/toth-itsbritishslang made a really insightful comment higher up pointing out that the reason we see Riley this way is because the writing is influenced by Buffy's perceptions, and I think that's absolutely right. I think that's what I was trying to convey in the OP when I said he's a "woman", but they said it much better and without all the gender politics tacked on.