I think they are great, but because statically generated sites can limit growth, just because if you want to add backend features (forms, commenting, etc.), you may have to switch platforms or use third party services.

I host my own blog on Jekyll and that works awesomely well and I have no plans for needing a backend server. If I was a startup and wanted to add small marketing features (email collection, surveys, etc.) then I would consider moving off of it.