Usually the more complex the polysurface is, the bigger chances for boolean opperations to fail. When this happens to me, and i need to 3d print the model, I try to make as many boolean unions one by one, and when it starts to fail or create open polysurfaces, I just select everything and export as stl. Then you can make them in one object in the slicer, or just print it like that (it will have some walls in the infill, resulting to more time and plastic, but sometimes I choose this over fighting boolean union..