When I was out tractor shopping, my dealer told me that the heavy brush hog I had my heart set on would be too much weight (front would lift) for my tractor without the FEL, even with a full front weight set. He based this concern on sections of the owner's manual - which he copied and highlighted for me - he also highlighted the part where it says that using unapproved implements (heavier than they recommend) would invalidate the warranty.
In addition, he stated that he had been raised on a farm and used similar equipment since he was a child and had sold Kubotas for 20 years. All of this he sent to me in a letter that I still have. Having absolutely zero tractor experience myself, I thought "What the heck does he know?".
So I did a quick moment analysis on my tractor. It was a real swag (Scientific Wild Arse Guess). First guess was the location of the center of gravity of the tractor with no implements. I assumed that it would be 2/5 of the distance from the front axle to the rear axle. Then I estimated implement weight & CG and threw in torque about the rear axle in 1st gear at PTO speed. According to my calculations, the dealer was right.
My analysis didn't account for height of the CG and I never did that estimation. This is important when you talk about ballasting and operation on slopes. Where the ballast CG is located, vertically, is also important; obviously, you want it as low as possible. Additionally, you want it fixed, so that if'n you do start to tip it doesn't start to move on you. I have always been a bit distrustful of the brush hog as ballast on slopes for this reason - with the flex link on top it can pivot about the bottom pins a fair amount, raising the CG as it does. A utility box filled with cut wood (what I use) can also be a risk - if there's so much wood that it can shift position or some can drop off the top, that's not good. A moving CG is a snake in the grass.
-Jim