My main reasons for a Stinger over a M340 (or even M440 Gran Coupe) were:
1) Size. I've never had a compact car for a reason (have always had midsize/large sedans). The Stinger and 3 Series aren't too different in size, but enough to make a difference. I'm 6 foot 2, and I drive friends/family pretty often. The back
seat of the Stinger is tight, but the 3 Series is just too small (same reason I didn't consider the
G70). I take road trips and generally need a decent sized trunk. Although I didn't buy the Stinger for the hatch, it has been very useful.
2) Price. People talk about buying Stingers at some crazy discount, but that was before Covid. I bought during Covid times, so there were no deals on Stingers or anything else for that matter. Any BMW 340/440 near me was $10,000-$15,000 more expensive than my GT2 (because feature for feature, to get the same level of equipment that's just how the BMWs had to be speced). Even if I could find a base 340, I didn't want to forge features I like/wanted just to get into a BMW. And I didn't really want to buy used (used prices were so close to new prices).
3) Somewhat unique. As other pointed out, one of the draws (for me) was that the Stinger is relatively rare (I know it's not like, collectable rare or super rare, but relatively). Relatively in that, Kia has only sold about 60,000 Stingers in the United States since 2018. Meanwhile, Dodge sells about 70,000-90,000 Chargers and 50,000-60,000 Challengers
every year, BMW sells about 40,000-50,000 3 series and 20,000 4 series
every year (not to mention every other BMW sold with BMWs ubiquitous styling).
4) Intended function. I was talking with a guy about the difference between a "sports car" and a "sport
y car." The Stinger was intended to be a grand touring car, not a canyon carving track monster, which is exactly what I use it for. It's my daily driver. I've put on almost 19,000 miles in the 13ish months that I've had it. I take road trips with it. I drive it in snow storms. I've had fun on back roads, but I don't want my daily driver set up uncomfortably for the few times I drive spiritedly. I'm sure the BMW handles better, I know it's faster, but I don't really need that enough to compromise.
As for the common complaints/criticisms of the Stinger:
Dealership. I haven't had any issues with any of the Kia Dealerships I went to. The forms and Reddit are filled with people bitching about Kia horror stories, but I just can't relate to any of them. I went to several different ones to test drive Stingers, all without issue. My buying process was fine (any car buying process is a little irritating). I did get $2,000 off the MSRP, even during Covid times (I was fine waiting for the right car and the right deal). My service appointments have been totally fine. I say fine because yes, it is a Kia dealership and not a BMW/Mercedes/Audi dealership. That's where my expectations are at. And the Kia dealers I went to were NO different/worse from any Chevy/Honda/Ford dealership I've been to (manufactures that also make and sell vehicles in the same price range as the Stinger) BUT, some people want to feel like they're getting the red carpet experience. Personally, I don't need that.
Quality Issues/"things going wrong". I'm not going to deny that the Stinger has known issues (brake shudder, sunroof squeak, rear hatch rattle being the most common). But I haven't had any problems with my car in the year + and 19,000 miles (knock on wood). The brakes are smooth (I followed a brake bedding procedure, maybe that helped?). My AWD all season tires still have plenty of tread left (some people mention the car eating through tires). My sunroof did start squeaking slightly this summer, but that went away with the heat (I'm going to try some silicone grease on the seals next spring if it comes back) otherwise no interior problems. My rear hatch has never made a noise. So, take that as my anecdotal evidence.
MPG. Nobody is buying a 340 or Stinger for it's efficiency, but man, BMW works some black magic on their engines. They make more power AND get better efficiency than the Stinger. People regularly can get high 30s MPG on the highway in their 340/440, and I think long term tests from MotorTrend and CarandDriver averaged high 20s. Whereas i'm averaging about 21-22 MPG (in mix of city and highway).
^ just my thoughts.