The GTSIO-520 is no easier nor more difficult to operate than any other Continental piston engine. Just like any other piston engine, however, they're subject to failure.

I've never flown the 421 or the 404. I have a fair amount of time in a Queen Air, they run geared Lycoming IGSO 480s. The primary thing that you wanted to be aware of was to make sure your props were always pulling. You do not want to push air with a geared engine IE flatten your pitch and use your props to slow down. I always treated the big geared Lycomings very gently with plenty of warm up time and gentle power changes. I treated them like they were big supercharged geared radial engines and I never had an issue with them.

It seems to me that I hear about an awful lot of those geared Cont 520s coming apart. In fact I don't think I know a single 421 operator who hasn't had engine problems.

So my original question still remains unanswered. The airplane was on a repositioning flight and presumably light (ish) weight. Will a 404 fly on one engine in Denver at lighter weight? The DA was way down with the temperature. Sounds like this was a sharp pilot. I'm thinking that he had multiple issues going on top of the engine failure.

Well over the years I have taken such a hit for giving speculations I hate to do it any more. But being the good sport I am, this airplane was almost certainly empty for repo to the UPS ramp at Denver, it is winter so density altitude was good, single engine climb on 404 is fine as long as the drag cleanup is done quickly. The airplane was probably well-maintained although I hear it sat idle for a few weeks prior to the crash, and it probably had ice in the fuel. I cannot recall how many sumps there are on the 404, but when you have an airplane getting used like this for repetitive 135 routes there is a tendency not to sump all the gas lines like they should. I suspect it had water in just about all the gas lines, and perhaps the pilot missed one or all of them on the preflight. I doubt it had cracked engine blocks, they would not allow that to happen and it probably had engine blankets installed on it. If ice in the gas was the cause, and I think it was, that also would explain why it lost both engines which is what I suspect happened. Just the same, the 404 is considered a risky airplane to lose any engine with at low altitude because of the geared setup as mentioned above. He really did not have a chance if that was the case. However, I find it perplexing that he landed in a housing area when Arapahoe Blvd. was right there. It looks to me as if one engine quit, he did the drag cleanup, then other one went and altitude was just too low to do anything else in time.

I visited the crash site yesterday and was given access to the yard via the homeowner. He walked me through exactly what the NTSB showed him. Based on the ground scaring, lack of wide debris field and the separated prop blades, this airplane was making power on the left side and was in a very slow VMC roll as it impacted right wing first. From impact to resting place the airplane cartwheeled about 100ft. ejecting the pilot still in his seat. He died on impact as no soot was discovered in his lungs.
If DS couldn't keep this airplane flying and back to the airport, I know I couldn't either. He was one helluva good stick.

The stuff nightmares are made of....

I guess my dual engine failure is not correct, or totally correct. Could still have been a partial power situation in the left if it was running on fumes and there was a fuel problem. I wonder if they can tell from the L prop how much power it was making.

Yeah for sure. If the left was at full power when this happened it sure makes the 404 look a beast in S/E situations. He may have let it get slow however, in which case the only thing left would be to reduce power on the good engine. It's either that or the fuel was running out due to ice clogging.

I've also learned that although the 404s were parked in October, this airplane had been flying regularly throughout peak on overflow charters.

The airplane was empty except for about 90 gals of fuel. Hard to imagine why he couldn't keep it flying. He should've been able to continue climbing single engine in those ambient conditions.

Track on flight aware was a constant turn to the right, not that it means anything (because it's several miles of distance) or is accurate.

These engines require extra care and feeding, particularly when cold. Was a proper runup, including cycling the props performed? The pilot indicated that he would be "ready reaching" makes me think otherwise.
A windmilling propeller will cost about 400 fpm, something else to consider.

I flew that exact plane for a while. During the short time I was there DS had an engine failure in that tail number. I'd be very surprised that it was just a single engine issue. They handled SE fairly well even with freight on board.

Speculation Warning!!! Maybe things changed since I flew there but I never heard of anyone actually getting deiced at APA while I was there. KLA did supply a small ice scrapper and some rags though. Not saying DS would fly a plane with air frame contamination but it could explain why the airplane was not able to hold altitude on one engine.