First, it takes over port 25 as your default SMTP server. This is great in that it saves you from dealing with the messy world of aliases etc, but not great on a shared host that does other things with mail as well.
Second, the FSM routing, while convenient, was ultimately limiting. Lamson routes mail based on the state of the sender (for example, 'subscribed', 'new', etc). But this state storage is abstracted away -- so if you wanted to change an address's state outside of the email flow (for example via a web app) or append additional data to the state, you have to re-implement the model logic for the FSM. I wonder if this is the reason why librelist, Zed's mailing list server implemented in lamson, has no web interface for subscription or list creation.
Lamson doesn't help you with parsing email any more than Python's standard library does (which is pretty decent, if a little verbose), so if you can handle setting up routing/aliases of mail to your application and storing state yourself, it might not add much.