I'll leave the interpretation of what "problematic" means to each individual, including if you think it means failures and quality problems getting fixed, but I am not really thinking about those types of problems but more any behavior when running or some characteristic that really, really annoys you compared to your other locos.
My most problematic loco is my Lionel UP 9000 4-12-2. It's a lovely loco, and I particularly wanted to have a model of it - as far as I know the prototype had the largest set of non-articulated drivers made. And that seems to be the problem: the very long set of drivers, even though many are blind (i.e., without flanges) makes it very sensitive to track issues. Mine definately requires every bit of its O-72 curve rating - I doubt it would run on O-70 or tighter, like many rated O-72 will. It barely seems to tolerate 072. When I had my Fastrack, I had built the layout using four locos to "prove" my track as I laid it out: an MTH scale Northern (2921), a JLC Big Boy and Veranda Turbines from both companies. I figured if those four would run smoothly on the curves I laid out, and tolerate smoothly changes in slope as they enter and leave climbs, I would have no problems with anything else. But Mr. 9000 had issues with two O-72 curves that were just a bit too tight for him, and there were two places where the slope changed from up to flat or flat to down in too short a distance, so that he would lift and clunk down and often push a flange off, etc.. I managed to modifiy a couple of spots to where it would get around the loop, but it still bobbled and clunked loadly in a few places. When I rebuilt with Atlas late last year, 9000 stayed on the layout nearly continually, and was the acid test of track smoothness, but it still makes me nervous as I watch some of its leading and trailing drivers go completely off the rail (sideways) as it negotiates a curve. Looks a bit strange, too, but then that's the price we pay for big locos and its not any stranger in appearance than the front of a scale Big Boy sticking out 3 inches.