Most of that piston check hauling was in multi-engine airplanes, though some single engine airplane operations were out there (210's, etc). Single engine flying in hard IMC is unwise for a number of reasons, not the least of which is engine failure, but also systems redundancy, equipment, and often, pilot qualification. That said, so long as nothing goes wrong, and radar isn't required, and navigation capability is there, it's often just down to single pilot IFR...which is among the highest of workloads one can have in an airplane.
On the face of it, the Mooney mishap strongly suggests pilot error: guy flies into powerlines. The periphery backs it up; small piston airplane, minimal equipment, inexperienced low-qualification pilot. Situationally, the pilot failed to follow ATC directions, and of course after the aircraft was wrapped in the power lines, admitted to the 911 dispatcher that he was "a little low" (no ****?). So, not a lot of surprise. That said, I'm a big believer in not making assumptions, so my first thought in looking this over was that perhaps his altimeter setting was off. This sets aside the failure to follow ATC direction or to navigate to the fix to which he was cleared (BEGKA). However, were that the case, his altitude error at each fix should have been consistent, all else being equal, providing that he did his part. Over BEGKA, he should have been 3,000', but was 2,775: 225' low. At TIMBE, he should have been at 2,200', but was at 1,500': 700' low. At JOXOX, he should have been 1,280', but crossed at 750': 530' low. When he finally impacted the powerlines, he came to rest at 600' MSL; the airport elevation is 539'.
The flight impacted 189' below minimums, one and a quarter miles from the runway, in reported visibility of one and a quarter miles: at impact he may have had his first chance to see the runway or approach lights, but likely not prior. However, before impact, he was at 475', which is 364' below minimums, and 64 feet below field elevation. He had to climb back up just to crash on final because he was below the airport.
Maybe the pilot realized just how deep he was in, or perhaps he wasn't aware at all; disorientation is implied, or at a minimum, a significant lack of situational awareness or insufficient core piloting skills, and quite possibly a combination of all. Regardless, it's quite evident that it wasn't simply a mis-set altimeter, and subsequent post-mishap testing of the altimeter system showed it within tolerance, and operating normally.
The totality of the information, even preliminary in nature, doesn't leave much room to question how the airplane ended up in the powerlines, but seems to point back to the same, tired realization; aviation continues to see the same kinds of mishaps from the same kinds of causes, over and over again.