Interesting convo. I started building custom basses a couple of years ago. I've also had a few custom biz experiences (on both sides of the fence) in my 62 years of life. My constant throughout has been to build in enough extra time when giving a completion date to a customer. If delays occur, typically there is ample time to compensate and still deliver on time. Yet, I can understand extraordinary circumstances that might push the date a bit. Absolute best case is to always deliver a little early - everyone is happy.
Now, the questions I ask are: What were the expectations for delivery? Were the specifics of your build in writing? How well has the builder communicated and provided photo documentation? Have you met your end of the bargain (i.e., making payments as detailed in your original agreement)? Has money been exchanged? How much? Have you documented/tracked your communication and grievances? Etc. I'm sure you see what I'm saying...things that occur in a pro business environment and endeavor.
If all is "up to snuff" otherwise, and the builder has simply been irresponsible and unprofessional, my opin is you have every right to request a full refund and terminate the build agreement if you choose to. At that point, the builder needs to honor your request and simply call it a bad deal. You, on the other hand as the customer, have the option to provide honest feedback (of some sort) to the public regarding your experience. At least that's how I'd play it... no need for threats of exposure or otherwise. It's business, not personal.