metal that the decks are made out of. thickness of the metal is one thing, but also the hardness of the metal. not all metals are made the same. a 1/4" of X type of metal. could be equivalent of 1/2" thickness of Y type of metal. metal has a bunch of extra specifications to it, that are beyond my own knowledge. but *shrugs*.
metal of the decks = rocks putting dents all through the deck, for say a light duty unit, vs a medium duty unit, a heavy duty deck metal going to take more punishment, without getting dents from rocks being picked up and through into the deck.
width of the cut = more width = more HP to spin and cut through material, and to keep the blades spinning.
some folks use rotatory cutters, not to go out into the pasture and around fields to cut that are 1 to 6 plus feet high, but rather sharpen the blades, much like a finishing mower deck, to mow the yard/lawn around the house, mowing the 1" to 6" grass for a yard/lawn, that has very fey if any rocks, or tree limbs = very little HP to run the rotatory cutter. vs cutting the tall weeds.
MPH (how fast you are going), more so geared tractors that have say 3 to 4 gears to choose from, has a great effect on how rotatory cutters do there job in cutting stuff up. if your traveling to fast, tractor bogs down to possibly stalling out, and not cutting weeds. going to slow (low gear), and takes a lot more time to use the rotatory cutter. generally the faster you travel MPH=miles per hour, the more abuse all equipment will take, including tractor, more so on rough terrain, and everything is bouncing, chattering away. in this respect you will more likely want something stronger and/or more flexiable, to be able to handle the abuse.
type of welds used to put stuff together has a difference of how well a deck can handle stuff long term. over all frame work of the deck also plays a roll in it.
location, location, location, i would imagine different areas of the world, and countries, even state to state, to county to county, to property to property is different. as in cutting down small trees, to how many rocks are in the area to be mowed, to finding that unknown tree limb that fell of tree and running over it, to finding unknown junk while using the rotatory cutter (ya don't know till the rotatory cutter hits it), and maybe not even then.
commercial use (use rotatory cutter for 100's of hours per year) vs a home owner that may put on 2 to 40 hours on it a year. does a home owner really need something that can handle running over a tire when mowing ditches along the road like road crews might need? or dealing with running over a stump? and not twisting the frame work, blades, etc...?
by default majority of all rotatory cutters come with a "slip clutch" but that is not always the case, and some come with a "shear pin" in the PTO shaft.
shock loading, "blades hitting a hard object like a large rock", that vibration from hitting, goes right up the gear box, through slip clutch... PTO shafts need to be able to handle that to some degree, or they will flex like a rubber band and bust / crack / come apart.
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The rest of the widgets, frame and wall gauges, etc, has nothing to do with what it takes to drive the blades.
your saying nothing else matters? i would have to disagree. it is a system of parts put together, it is not the individual parts that makes it all up and works it, but the overall system of parts and how they are put together and how they are used.