The problem from what I've read is most ads are inserted on demand in the stream when you first play/download a podcast and there's no easy way for software to tell them apart, not to mention they're usually location based which means the ads you get are likely different from what I get, and depending where you live you might not even get ads at all.
I use antenna pod which is able to download as well, which seems to suggest that at some point the ads get fixed. Also they're very consistent for me, enough that I can usually cut off the first 30 seconds of ads for certain podcasts (there's a function to skip predefined sections).