I've been toying with constructors by adding multiple constructor modules to my designs. I very much like the idea (it can speed up starbase construction and development). Unfortunately there are few problems I've noticed.
First, if you have more construction modules on the constructor than the starbase has modules that could be built, it is likely that the constructor would not add to the pool of construction points, but instead fortify the starbase like any other ship. It might be because of some piece of code intended to avoid waste, but it doesn't work very well. I would be fine if the starbase wound up with more construction points than it could use. It would save me the trouble of sending more constructors later.
The second problem is when I try to built a starbase using a constructor with multiple construction modules. The excess modules end up being wasted. I might as well have used a normal constructor.
Another problem I recently considered (but haven't tested yet) is if there is also waste if I use a constructor to build a shipyard. There probably will be since shipyards don't have any use for constructor modules.