Originally Posted by jonp
100kg on the hitch is only like having one average sized person these days sitting in the boot or two bags of cement.
Its not! Not withstanding the fact that an average man
is 83.6 kg (in the UK, 2010) having the weight in the boot (probably directly over the rear axle) is rather different from having it positioned on a lever perhaps a meter behind the axle. Perhaps even more importantly passengers or cement are sitting pretty still in the boot, whilst the tow ball load is dynamically changing as the trailer passes over bumps etc. Of course the manufacturers spec assumes you also have your car loaded - which in many cases with people going away for a long trip it will be! It is unusual to find "family cars" in the UK rated to take 100 kg nose weight, even though they may well be rated to pull 1600 kg.
Ultimately though, even if the car can take it without the handling being affected you can expect that if it falls under scrutiny (either by an insurer or the authorities as the result of an accident or a random* stop) then being massively over the manufacturer's stated nose weight limit isn't going to be good.
Finally from a purely practical point of view anything too heavy becomes impossible to lift on and off the ball which makes you reliant on the jockey wheel (which we all know are liable to jamming under excess load) and harder to manage when moving around without the aid of a vehicle.
* And having a car that appears to be sitting low at the back because of a heavy trailer is a good way to get selected 'at random'.