Please read my previous post on the front axle strength.
The numbers will show that the front axle is not safe for extra vertical loads from add on implements applied at the center axle pin hole for load over 400#.
That is the basic reason.
Imagine the result ,if the front axle fractured from lifting a load of gravel with the bucket in the fully raised position.
In my opinion the front axle is not inherently weak for the original purpose of the tractor.