As discussed on discord.

Right now while it's possible to add fluid output slots, they won't work due to the fact the multiblock behavior is hardcoded. Adding more slots would also require adding another layer to the structure.

Same for adding an item output.

I'd need to do the same on a distillery, but I guess in that case I can create a new machine for the purpose. Single block machines can't mess with slots, but also don't have custom behaviors like the distillation tower.

I was also wondering if there's some reason behind setting certain machines to have limited fluid/items input/output.

It seems redundant because for all machines the slots are defined again both by UI and recipes. Wouldn't that be enough and skip entirely the registration of recipe type, so that it gets automatically defined for the whole range? If then a machine doesn't use fluids, it will just show no fluid slot.

I guess a multiblock would form with the "wrong" hatch, but it seems a minor issue if it's only that.

I was also wondering if there's some reason behind setting certain machines to have limited fluid/items input/output.

It was added to prevent datapack authors from adding broken recipes. This was a very long time ago, when the KubeJS API didn't exist yet.