They are a publisher.
Their individual IPs are headed by completely different dev studios.
I'm pretty sure most of their flagship bi-annual IP are actually made in collaboration with their studios working together. Either way, they own all of the studios that work on their games, and I think most if not all of them even have the Ubisoft name. It seems like they run their studios a lot more like say Nintendo EPD is run, rather than say Retro or Monolith to Nintendo, only with entire studios instead of teams within a larger studio.
Also they own the studios so they're really a developer and publisher. But if you took the OP to only mean single studio developers, or developers who can't self-publish, I understand.