I haven’t looked it up, but assume this phrase comes from the fact that cast iron is very strong and therefore hard to break.
Having had many encounters with cast iron over the years I can confirm that it is strong, but also that it rusts over the years. In the case of gutters, many proud house owners will paint the exterior and seen from the ground they look great! But as a rule they will not check the interior, invisible from the ground, where the water flows and also where it stands for long periods. Why does it do this? Sometimes the gutter is not completely horizontal on its way to the downpipe, other times it faces obstacles such as autumn leaves or, very often, grass and other plants finding a wet gutter offers a happy home.
It’s when things go wrong that we discover how hard/impossible cast iron is to work with. There is absolutely no give in it and it is also brittle and easy to break if you try too hard. It can be cut with an angle grinder, thoough. The most I have ever been able to do is patch it, provided the hole made by the rust is not too large.
A case in point. On my way to visit my daughter I pass a house with a cast iron problem. As may easily be seen most of the water does not make it all the way down the pipe, and the wall won’t like that. On a rainy day, neither will passers by.
But even if it did make it all the way to the ground, it hits another problem.
How did this pipe end up in such a bad condition? I don’t know, of course, but I can guess. It may have been where it is now since 1897 and age has taken its toll. (Know the feeling!) Another possibility is that the drain is entirely or partially blocked below ground level. This will mean water filling the downpipe for hours, days , weeks, because it can’t run away. And this will lead to rust.
For this here pipe there is no hope. And if you’ve just bumped off your husband or wife, the same will be true of your cast iron alibi.
As for cast iron guarantees . . .