Which was it? Marginal VFR? IFR? The visibility that night was 8 statute miles. Hardly Instrument Meterological Conditions.
And Martha's Vineyard was lit up like a christmas tree, according to witnesses.
JFK Jr had about 300 hrs total time. 40 in make and model. And that make and model (Piper Saratoga II) and it's fore-runners, the Piper Lance and Cherokee Six are among the most stable instrument platforms in single-engine aviation.
He had completed instrument training, and needed only a checkride to be certificated, i.e. Instrument rated.
No "black helicopters" here ;)
The Vinyard may have been "lite up like a Christmas tree" --but that matters little if the visibility is that low due to haze and other factors. I have landed at Frankfurt International when it was "lite up like a Christmas tree" and I barely made it in there with Cat II procedures.
The stability of Piper aircraft is not in question--but when you lost situational awareness and have spacial disorientation in an aircraft without relying on your instrument, you get the "graveyard spiral"--which most investigators believe happened.
Even 5000+ hour instrument pilots have mishaps. JFK didn't even have the rating.