What's the most frustrating part of monetizing a newsletter?

If you run a newsletter — even a small one — I'd love to hear: what's been the most frustrating part of trying to monetize it?

I keep hearing the same few things from creators: ad networks that require 2,500+ subscribers before you can even apply, sponsorship deals that take weeks of back-and-forth for a single send, or "programmatic" options that don't actually work well in email because they're really just web ad tech wearing an email costume.

Curious whether that matches what others have run into — or if there's a different pain point I'm missing entirely. Building something in this space (MailAdx) and want to make sure I'm solving the real problem, not just the one I assumed existed.