Actually got to thinking about this a bit.
I looked at the keeper I built to hold a stop pin in on the breakaway wing springs on our sprayer, used leftover 409 pipe from my waste oil boiler, the piece of 1/4” mild steel flat is rusty as can be, the 409 is still shiny.
Looked at the electric transfer pump I put together a few years ago to handle 28% and 32% UAN, the stainless age iron frame is shiny, the mild steel bolts holding the 10hp electric motor and pump to it will either need cut or broke off, they ain’t turning.
Looked at our fertilizer spreader, this things gotta be 40 years old at least, it was used when Dad bought it and I was still too young to spread fertilizer so maybe 50+ years, the stainless box is fine, the original rear assembly rotted away years ago, we replaced that with stainless, still shiny, it’s spread thousands of tons of fertilizer over the years, replaced all the roller chains with stainless as mild steel ones would last at best two years before rusting solid. Have replaced all the leaf springs a couple of times, fertilizer even eats spring steel, have replaced the axles once as well. Have replaced all the bearings with stainless ones as well as regular steel bearings would last at best two years.
This is hypothetical at best, but if a person had enough single wall pipe on it like say if they had to meet one of those arcane minimum height requirements, you could get a lot of condensation, that could mix with the flyash and literally make mud inside the pipe or even gum up the exhaust fan.
But when it comes down to it, doesn’t natter if your buying a car or a boiler, by buying it your basically agreeing to follow the manufacturers policy on best practices to maintain your warranty. If they say an oil change every 5000 miles at a certified dealer, then thats where you get your oil changed, if they say X kind of pipe and the wood should be under a certain percentage, then so be it. Ditto on keeping it dry and avoiding having ash and moisture mix.