It is not quite that simple. Machinability does not mean the steel is strong or weak. You have to look at elasticity, elongation, yield strength, hardness and the method of processing before deciding what material is best for a particular use. Stainless is great for our crash bars, partially because it won't rust. But many kinds of stainless would not be as tough some plain carbon steels. I love my stainless steel knife blades, they would be almost impossible to drill with a regular bit, hold an edge okay, but if I want a really sharp tough knife I use a carbon steel blade.
Good old Wikipedia puts it well- "Stainless steels have poor machinability compared to regular carbon steel because they are tougher, gummier and tend to work harden very rapidly."
Or if we go to aluminum, we give up some toughness, but save weight! Not so simple choices. If you really want to look at materials, check this- http://www.engineershandbook.com/Tables/materials.htm You can see the numbers overlap a great deal, depending on which steels you compare.
Stainless, especially higher grade 300 series, are tough to drill in part due to their "softness" The initial attempt to drill the stuff causes material to smear about the divot your making with your drill bit and then work harden. The bit never gets much chance to bite into the material and just spins around on a combination of soft gooey material as the bit advances, whilst fighting the thickening hardened layer being built up in its path. You kind of end up just trying to abrade the material away instead of cutting through it. As pointed out above, this is not related to the stiffness of the material. I would think for crash bars you would want a very high strength material to resist them bending into and contacting the bits you're trying to protect. Better yet to combine that with high elastic modulus to resist permanent deformation. Maybe a high grade chrome moly material?
The impact on cooling can't be as much as the ADVRideOn ones, so I'd not worry about that.
The mounting is a lot like the ACD in picking up the engine mounts in front and the swingarm pivots in the rear. The aft left mount isn't going to take as much of an impact but still looks OK.
The front right looks like it is more to protect the radiator hose.
The problem with plates that have access to oil filters and drain plugs is that the oil still dribbles inside and it's a pain to clean.
You might think about cutting the boss at the front of the oil pan if you want to do real offroad. From this different plate striking the boss:
You can cut at the upper threaded area to minimize this risk, but NO HIGHER. The interior of the pan has a low spot in this area.