Greg, I pull a 13-yd scraper behind two different MFD tractors. One has 215 HP one has 185 HP (PTO), I usually stop loading when the dirt is running off the sides. There is no way that either one of these tractors has anywhere near the traction that a D6 or a D7 has. The 13-yd scraper that I pull has the same size bowl as a Cat-70. but the angle of cut is not near as steep. There is no way that I could ever load a Cat-70 but I would have no trouble pulling it loaded anywhere that I needed to go.
A 200 HP MFD tractor, in my opinion and in most dirt, has more trouble loading a Cat-60, than it would have loading most modern 13-yd scrapers.
Yes, I have seen some sub-soils that a MFD would struggle to load. But, in the 4' deep cuts I was making this fall for a diversion, I was still able to load a 13-yard scraper. Keep in mind that north central Missouri had one of the driest late summer and falls on record. The clay was very hard and I'm sure some brands of scrapers I would have had trouble with going into the ground, and some other scrapers would have pulled too hard for me to load.
Although I have no trouble loading a 13-yard in Missouri gumbo, I have seen some gumbo's, such that is in some places in Minnisota, that I would struggle in or not be able to load.