I think it kind of depends on the scope of the project, and how public those demos are. For example, my 4th ending mod only took about 5-6 weeks to make before it was mostly done and ready to enter beta testing, so there was no need to release a demo along the way.
If the amount of time needed to make the mod is much more than 6 months, then I think there's a lot of worth in releasing a demo before it's done, because waiting all that time with little to no community engagement can be exhausting. The feedback and bug reports also help a lot with giving the mod a better sense of direction. Of course, there might be reasons why you want to keep the project under wraps until it's done, in which case you can release some private demos within some exclusive group of Cave Story fans that you know.
If it's a more time-consuming mod, I would recommend releasing one or more demos throughout development unless there's some super practical reason to wait until it's done before releasing it publicly.
If the amount of time needed to make the mod is much more than 6 months, then I think there's a lot of worth in releasing a demo before it's done, because waiting all that time with little to no community engagement can be exhausting. The feedback and bug reports also help a lot with giving the mod a better sense of direction. Of course, there might be reasons why you want to keep the project under wraps until it's done, in which case you can release some private demos within some exclusive group of Cave Story fans that you know.
If it's a more time-consuming mod, I would recommend releasing one or more demos throughout development unless there's some super practical reason to wait until it's done before releasing it publicly.
There's also a chance that if you release it entirely finished that nobody cares lol