Code-heavy
Inbound email actions require Javascript experience. Developers have higher priority tasks, so important email needs are often backlogged.
Difficult to Test
Changing a monolithic inbound email action can cause unexpected issues. Full regression testing each time you add an email rule is unwieldy.
Difficult to Manage
Organizations need to add/remove email rules all the time, and there is no safe way to make changes on-the-fly in ServiceNow.