They are certainly in the "best bargains out there" category, like so many other Guilds.
I don't know for certain why other people don't rave about them, or why they aren't valued higher, but I can offer a guess about why they sit where they do in the market and also explain my own thinking on the Bluesbirds.
Let's start out by agreeing that the setup and look of the 90's BB generally belongs in the "Les Paul copy" category of guitars. I don't think that's stretching things too far. In fact I'm pretty sure it's predecessor, the 70's solidbody M-75 was created in direct response to the popularity of the Les Paul, the same way the S-100 was a response to the look and popularity of the SG.
What I do know is that in nearly every industry, if you release a "me too" product, you will almost NEVER get the comparison to the original off your back, and it's almost impossible to avoid the take away: "not as good as the original." This is true when you're bringing out a product that is supposed to compete directly with a competitor's already popular product, and it's also true when you reissue a product of your own. Doesn't matter what the "real" facts are, the thing you are going to hear over and over again is "it's not as good as the original," even when that's not true.
Secondly, these guitars are not quite 20 years old. Too old to be considered "new" but too young to be considered "vintage" that means they are at a no-mans land for collectors.
Long story boring, if you're into Les Pauls, you'll probably just go for a Les Paul. If you're into Guilds, you'll probably go for a Guild that has classic features that set Guilds apart from other brands (Guild pickups, etc). if you're into old guitars, you might not aim for this, if you're into the latest things coming out, once again, you'll probably skip the Bluesbird.
All of this, of course, is a shame. These guitars are fantastic bargains, especially for being made in the US.
So what about me, personally? After all these years of collecting Guilds, a 90s Bluesbird never made it into my "permanent" collection. And the reason has a lot to do with what I said above. It's a Les Paul-ish design. And honestly, I'm just not that into les pauls or duncan pickups. The 90's Bluesbirds just don't do it for me. But of course, now that I've been writing about them, I want to get one!