Take the HU out of the equation, then, based on all of the time I’ve spent around here since buying my 2015, I’d say there’s really no statistically valid difference. No “bad” years for the Gen 5 3.6R mechanically. Just consider the usual stuff like mileage and maintenance. Obviously, a 2015 has been on the road that much longer, but also consider what part of the country it’s from. Also, 2015-17 have HID headlights, 18-19 got LEDs, so that might be something to consider. I’ve seen very few HID problems reported here that weren’t easily fixable (actually, I can’t think of any) – LED units seem to have more problems and they’re not really serviceable. I think a replacement LED projector assembly is even more expensive than the HID.
Regardless, as mentioned, I’d have it inspected by a 3rd party before purchase. In addition to CVTF weeps, the EZ36 engine itself, at higher mileages, has a tendency to weep oil from a few different places. Some are easily (and cheaply) repaired, others not so much (but those are often so minor that they can be tolerated).
