That would depend a great deal on the type of aluminum that Kubota and others are using.
As a retired pattern maker who has made patterns for most of the larger farm implement companies including John Deere. Ford, International, White, Steiger and esp. Versatile I also have some misgivings about the aluminum rear ends on the smaller tractors. However as I said it depends on the aluminum alloy being used and the misgivings are not so much about the strength but rather one of weight, (I happen to like more pounds on the ground).
As to the strength of the castings, if Kubota is using an alloy like 60-65T5 or at the upper end 70-75T6 there is NO problem as these are the types of aluminum alloys that are used in ALL aircraft landing gear and other high stress structural airframe parts , and I do mean all, and these parts go through more stress each and every time the aircraft touches the ground than your Kubota will ever see. Aluminum can be made HUGELY strong but it all depends on the alloy that is being used if it's just beer can aluminum,well that ain't so good but somehow I tend to think that Kubota is using some of the upper end heat treated alloys for their differentials.
The biggest concern I have with using Aluminum for the differentials is one of sound transmission esp. with HST's. Cast iron deadens sound while aluminum shall we say does NOT but thats what ear plugs are for.
All in all I wouldn't worry about it to much, but like I said "it ALL depends on the alloy being used". Perhaps someone else knows what that alloy is.
