I agree the CTS is a dark horse. Everyone likes the styling, which I think is first-rate. But beyond that there are various factors holding it back. Many may pigeon-hole the CTS in the historic 'Grandad's Land Yacht' category, not realizing the CTS is truly a sporty performance drivers car. Similarly, I detect some people have kind of a '1%' aversion to the brand (which somehow doesn't stick to much more expensive foreign cars?) - personally don't care what they think. Yes, there's deserved concern about reliability & service, but I'm not convinced it's 'way worse' than other brands. More features always equals more problems, expensive cars always cost more to fix and finding good service is always a challenge for every brand (indeed, the car business in general IMHO). I'm not downplaying/unsympathetic to the horror stories we see and 'dealer service' can surely seem like an oxymoron (for any brand). Finally, like the Corvette, CTSs (at least were) kind of a bargain for what you get in features & performance (and I'm a bit dubious about Cadillacs recent strategy to boost the price a lot). And, ever hopeful, I try to buy American - and it's worked out OK.
The good news is the markets overlooking the CTS and the (once lowish) sticker price means there's a chance to get a great deal on a used one. Go into the dealer and hit 'em with 'Might give you a few bucks for this used old farts car, if it still runs har-har, how much?' :=) Sure it's a gamble, but if find a good one it can pay off - world class styling/performance/features for less than you might imagine.
It's interesting that Cadillac had a big resurgence in the 2000s mainly due to the new styling cues and performance of the CTS and similarly designed models. Sporty, sharp, angular were distinct and a new direction. In my opinion the sales failures are because they lost their vision, rounded out the styling to satisfy a global audience. The "Grandad's Land Yacht" microfuture mentioned is not only historic it is a reality that Cadillac is moving back to, at least in their current design cues. Performance is still there but its hidden in the design. I personally think they have lost their way and with it their imagination. One of the reasons I bought a 14 CTS Coupe instead of an 15 ATS or CTS sedan.