The nicest / most surprising thing to me was the instant image preview. In less than a second I had 'uploaded' a huge image and was editing. Nice work.
I felt like Backbone.js (when I was actively using it) had really clean and complete docs. Couple that with the fact that the source is incredibly readable and debugging anything strange was never that tough.
I think you're pretty spot on in your analyses though. We use static sites for our landing pages (currently using Jekyll, Grunt & S3) and I can't see a real reason to switch to something like this.
From a business standpoint I worry Forge will have a really hard time building a sustainable business off of $10 / month / user. That's a lot of users they'll need to build something profitable. There is likely room here though for add-on services or some unique features.
As a fiduciary you'll need to add a lot more content to your landing page to build trust. While the idea sounds great you're asking someone to put a lot of faith (and there paycheck) in your hands.
I'm also curious - what's to stop a client from never releasing payment? Who resolves disputes?
I think you're pretty spot on. A PR agency might pay 10x times this, but for a one off use by a startup or SMB a monthly charge doesn't make sense. Maybe offer pre-paid credits as well?
How many signups have you see / are you expecting? In my experience, it's much easier to open the flood gates and close them if need be than the other way around. Remove all friction and just get users in.