Hydrophyl is right. There isn't a vac that will clear every blockage, especially a tough weed in a downpipe.
Fortunately these are quite rare but I still have ladders just in case ....
... and then theres the customer who asks you to "just pop up and fix the downpipe" that has come out of it's housing.
I'm happy with most of these jobs. But I know my limitations. If I don't like the look of it I get my mate the roofer to do it