If anything were to fail, it would be the bolts holding the tow bar to the car. These are usually 8 or 10 mm bolts and far seeker than the 16mm bolts on the tow ball. I grew up in the motor trade and as far as I know a tow ball has never broken other than in an accident. I have seen the hitch catch fail on a few occasions and the hitch detatch from the ball though.
Although very rare I would think that the hitch coming away from the tow-ball would be infinitely more likely than a tow-ball breaking. They would normally last far longer than the car, I would have thought.
In all probability far in excess of 50 years, unless wear took its toll. Wearing out would be far more likely than breaking anyway.
The towball will be fine - if there were any safety issues there'd be caravans and trailers littering our roads and a national scandal.
One thing the EU gave us was Type Approval, meaning towbars and towballs need to be fit for purpose and properly designed and manufactured. Before this people were free to weld some Heath-Robertson contraption to the back of their vehicle.
Al-Ko do good stuff, but it costs three times as much as equivalent stuff from other manufacturers. But at the end of the day £15 or even £30 is neither here nor there in caravanning terms (just had my annual service) so if it gives you peace of mind I would go for it.
------------- Camping Gear expands so as to fill the space available for its transportation.