I demo'd a D-20, probably 15 years ago. We were looking for something small to replace the ole' 450C Case. We did a lot of channel work with the 450, and placed a lot of stone under bridges, where flooding had scoured the abutments.
I didn't care for the direct drive. I was doing some finish work on a job, and snagged a stump, no more than 2"-3", that the larger dozers had missed. It literally threw the dozer sideways, and killed the engine, as I was grading with the blade at approx. 30コ angle.
They are pretty light, in comparison to HP. It seemd to have plenty of power, but too much for it's weight. It would literally power out (spin), if you tried to cut hard dirt. Possibly a winch on the back may help that issue.
Guess I was just used to the Case's we had, the all did have winches on them.
I could see it's use in a housing project, finish grading loose dirt, in close area's, but not much more than that
They may not be top of the line, but I liked the Case dozer's, simply for the fact you can power each track separate from the other. Both in low for power. One in neutral for a slow turn, or one in high to make a power turn, not just feathering, or killing a track with a steering clutch. Both in high, to tram back on a long push, at lower RPM's, or just to get across, or out of a soft spot, to get torque to the tracks, not power.
Getting aggresive corner bits on any machine makes a world of difference. Angling, and tilting the blade, making it act somewhat like a plow can rip some mighty hard stuff, once they are sharp. I have ripped a lot of semi-hard shale rock with the 450C. The 850 models we had did better.
The only drawbacks I saw with the older Case dozer's, was the dry brakes. Being we did a lot of work in streams, water and sedimant got in the brake housings, and rusted the steel balls in the brake actuators.