There was some truth to this 20+ years ago. Of course all normal farm tractors that were not built to handle a front loader suffered from this. Remember, a standard farm tractor is built to handle and tow implements, NOT a front end loader. Now that we have all decided how handy a FEL is, we all want them and it was a learning curve.
Just a look at old farm tractors with loaders, they all have very small "manure" buckets and manual trips almost to a tee, to try and keep front end overloading to a minimum. How many 8N, 9N's do you see with a three to three and a half foot manual trip bucket, back in the day, lots.
You tube is full of videos of front axle failures on all makes of tractors.
Rest assured though that Kubota learned its lesson and now builds a loader matched to a particular tractor model that won't overload the front axle, if the proper recommended ballast is on the three point. Part of the learning curve was a heavier duty front axle as well.
David