From my personal experience the biggest single factor in flying in IMC when not trained, qualified or experienced in doing that is huge and unjustified over-confidence in one's own ability. This applies to people under about 40 years old.
Paradoxically, the IMC Rating, in my day, and I guess the IR(R) today, had/have the dual effect of improving PPLs' ability to fly safely in IMC, while increasing their unjustified belief that they can do it perfectly, even when things go slightly wrong.
(In my case, "things going slightly wrong" was an engine failure (of the only engine) while climbing through cloud to "VFR on top" over hills, which was a challenge I felt quite unprepared for, even though I had an IMC Rating.