Okay, a few things here...
First, scrape vs. shear. A scrape is a cut where the cutting edge is at 90 degrees to the spin of the wood, and best example would be a scraper flat on the tool rest. A shear cut, bevel rubbing or not means the cutting edge is at an angle to the spin rather than at 90 degrees. Generally the higher the shear angle is, the cleaner the cut because the cutting edge is able to lift the fiber more gently as it cuts, so think of one of Doug Thompson's fluteless gouges at 70 degrees compared to most gouges which can only roll to 45 or so, unless you have a swept back grind and drop the handle really low, which you can do only on the outside of a bow. On end grain, like boxes, you can get a good finish cut with a scraping cut because of grain structure all going the same direction as long as you are cutting down hill. With side grain, a scraping cut will not give as clean of a cut as a shear cut. Twice each rotation, you are cutting against the grain, and a scraping cut with either gouge or scraper will pull more than a shear cut. Makes no difference if you have a big burr, or a tiny one, regular scraper, or NRS, all scraping cuts pull at the fiber. So, on bowls, scrapers are fine for sweeping across the bottom of the bowl, and a tiny way through the transition and a tiny bit up the side. Go too far, you not only start pulling out fiber, you can start the bowl to 'screeching and howling and then it blows up'. A scrape works across the bottom fairly well because you are cutting across the grain rather than going down through it. Softer woods will tear more than harder woods.
Now, burrs... I prefer scrapers for all of my shear scraping, and I always use a burr. More mass to them when compared to gouges, and I just like scrapers... Different ways to raise the burr: straight from the grinder, honed, and burnished. There are huge variations in each. I have noticed little difference in the metals, but huge differences in the wheels, especially when comparing CBN to standard wheels. These can also vary a lot depending on whether you just kiss the surface or push it into the wheel. As near as I can tell, with the CBN wheels, you get an almost honed burr (very strong and suitable for heavy roughing), rather than the wire edge more common from standard wheels (very weak). The finer the wheel, the finer the burr, with 180 and 80 grit wheels giving an edge suitable for heavy roughing and pretty good for most shear scraping. With the 600 and 1000 grit wheels, you get a very fine burr suitable for fine shear scraping and not good for heavy roughing.
When honing a burr on scrapers, usually this is using a diamond card, and pushing either up or parallel to the bevel. Most of the time this is a very fine card, and these burrs are more suited to fine finish cuts, and not good for heavy roughing cuts. For shear scraping, you want fine. For the McNaughton coring blade tips, you want a coarser, like 220 card for honing since it lasts longer.
Burnishing is another method to raise a burr. Grind, hone off the burr, or as much as you can, then use a burnishing tool (hard steel rod like for card scrapers, or carbide tipped, either hand held or the one from Lee Valley tools which bolts to the bench). They can be used on standard and negative rake scrapers. The advantage to this type of burr is that it is supposed to be stronger and sharper than the burr from grinders, and I have to experiment more with them. Mostly in the past, I couldn't tell any difference. I saw a demo once where the turner stated flat out that it was impossible to hand burnish HSS. Being skeptical, I went home and tried it, and with minimal effort, I could do it. He was really cranking on the Lee Valley burnishing bench mounted tool, and probably raised a tsunami type burr. I was using the standard hand held ones intended for card scrapers. Anyway, a very light burnishing, with the burnishing tool at maybe 10 degrees to the bevel (like 70 degree bevel, and burnishing tool at 80 degrees) you can produce a very fine burr. This burr seems to be pretty good for roughing, but not superior to the one from the 180 grit CBN wheel. More than anything to me, especially since I got the 600 and 1000 grit wheels, the extra work of burnishing just doesn't show any real improvement of the cutting edge to me. I may have to go back to HSS and experiment some more.
Burnishing the NRS... I saw Eric Lofstrom recently, and he burnishes a burr on his NRS. Hmm, really having to think about that. The NRS is still a new tool to me, but I made a burnishing tool to experiment with about an 1/8 inch carbide tipped rod in the end and uses that. He seems to get good results with it on standard use, and it does seem to hold up for roughing. I have tried it on both 30/30 bevel NRS, and 70/25 NRS, and get good results, but can't really tell if I get better results than with standard off the fine wheel burrs, but they do seem to last longer. Tom Wirsing, who is a master with the NRS does not agree that the burnished burr works better. He and Stuart Batty will be doing a demo at the Symposium with a 300X camera on burrs and edges, and that is on my agenda as well, especially to compare the different grit wheels, and new vs. old CBN wheels...
So, more testing needed...
robo hippy