Hi David, yes this is a bug in the surface/surface intersection calculation (which is one step of the boolean process).
The geometry library that MoI uses sometimes has a problem calculating the intersection between 2 tubular surfaces that touch in a single "crown point" like you have here.
Since the upper part works, to finish this particular model you can cut the pieces in half, then perform the intersection on the halves, and then mirror that to make the bottom half and union those together, I have attached a version here rare_top_half.3dm which has that set up, in case you need to do this to finish up.
This needs to be fixed inside the geometry library, so it may take a while before this bug is finally fixed up.
 Michael
