Jon Lunn shares some thoughts:
Fabric User Data Functions are a bit of a strange hybrid. They can be used like Azure Functions, to call some code outside and act like a serverless compute function, but also can extend notebooks as a source of reusable data logic. I’ve been using them since preview, and now they support Azure Key Vault (AKV) and Fabric Variable Libraries, I think they are ready (mostly) for productionable use.
I think the part that annoys me the most about them is that they’re called “User Data Functions” instead of “User-Defined Functions” and yet the acronym is the same.