When you hover over a factory building it says it has zero construction area and so forth. Also it gets a dashed line to other roboports. What's up with that?

When adding new types of buildings, I can't just create a completely new building type, I have to choose from existing building types, and no building type is perfect for the job. I chose the roboport building type, since it has the qualities I need and its drawbacks are purely visual. I suspect I could change it to the accumulator building type, but changing the building type would invalidate all existing placed factories so I need to be very careful if I want to change that.

Is it impossible to create your own building type?
Or is it just very complicated?

Building types are defined in the C++ binaries, and mods only have access to the Lua API. So the least complicated way to add a new building type would be to ask the Factorio devs if they could pretty please add a brand new building type into Factorio just so I can use it in a mod.

Any buildings having names (backer names) are good for Factories, because you can name your factories using Renamer mod. So you can identify your factories standing on the ground.