Sparks and old birds nests !! Thing is, it could be smouldering away for quite a while before the roof goes up. Hammer and a 2" chissel is what i've always used,not for repairing mind,just when taking them down to replace with plastic.
In terms of collecting rainwater and putting it a soakaway, i'd say plastic does a fairly good job.
It is also easy to work on, bits can be replaced and it requires little maintenance.
However, it does fade and is nowhere near as robust a iron.
Iron guttering requires a fair bit of maintenance, that in all honesty, rules it out of 98% of all homes. There are not many people who would chose a roof product that needs annual care. They are more likely to opt for say 10 year replacement of plastic.
There is a plastic gutter available that mimics iron. Looks ok - ish.
I have been quoted for replacement but it was way too much, also the guttering is fairly good (I have painted). Leak is at a joint in the cast iron and also where it joins my neighbours 'plastic' gutter. I have tried a few companies to get fixed but no one seems interested in trying to fix. I am not looking for 10 year gaurantees or anything just stop it dripping on to roughcast and potentially soaking into the wall. Any takers?