I would add the skinny 3/4 or 1" axle shafts on most "tractors" wont like the added stress of adding wheel weights, and fluid in the tires,even if they do only spin at 8 mph at the most,you'll all have to agree the "flywheel effect" WILL add considerable stress to the axle shafts..a lot more!..
I'd say once you get 100+ lbs spinning,even slowly,it'll be MUCH harder to stop than something weighing only 25 lbs would,and will generate a LOT more torque than a unloaded tire & rim with no wheel weights--plus they act like a gyroscope in a way,any imbalance or wobble gets eggagerated greatly,with all the added weight..I've seen axles snap off from the torque of repeated spinning and grabbing when used on an icy asphalt driveway to plow,with wheel weights,loaded tires, and chains..even without any "extra"weight added to the tractor itself..
I agree the cast aluminum transaxle housings wont like having much over 600 lbs total on them too,after seeing how thin they are, when I busted one in two when a tractor fell off the ramps ,when I was loading it into my truck!..the old cast iron ones were weak enough,but MUCH beefier than todays aluminum ones are...
I feel they use much too wimpy transaxles (and chassis!) on many of todays machines,give me the old cast iron ones with hi and lo range anyday..how they expect them to hold up for any length of time if used for dirt or snow plowing with lots of weight on them is beyond me,it's asking for failure,IMO--.
-..that said though,I HAVE been surprised at the amount of abuse a few "wimpy" transaxles took I have had in the past, without failure,but I never put wheel weights or loaded the tires either,just used chains...but more than a few failed wihout much abuse at all too!.
Why they dont just use automotive type trannys like an Economy tractor I dont know,it wouldn't have to be a 1 ton truck one to "live" behind a small engine with 25 hp or less..they aren't much costlier than todays aluminum transaxles are really,in surplus catalogs I see them selling for 300+ bucks!..I've seen brand new Saginaw 3 speed GM trannies sell for about 300 bucks in years past,one of those would probably never fail,if it can pull a 3500 lb car around for 100,000 miles..and making a narrow version of a typical RWD car differential wouldn't cost that much either,once it's tooled up..
I'd think a manufacturer would want to use quality drivetrain parts and get a good reputation going,for ruggedness and dependability,like the Gravely's and Economy tractors--but I guess todays way of thinking is to make it disposeable,no matter how costly it was to buy new...I dont get it!...
This is why I cob my own tractors together from scratch,to use to plow or tow stuff,out of old car parts..for peanuts you can assemble one that will make most of todays tractors look sick!..(One good thing about "disposable" ones though,is they ensure a good supply of good used motors,pulleys,wheels ,etc!,to use on your homebuilts!).