Ahhh I figured it out. I tried to cut shapes out of my mesh, after I filleted some edges, so MoI wasn't able to properly calculate the cut for those rounded edges. I recreated the mesh, made the cuts, and afterwards did the fillet operation.

That's great Groot, I'm glad you figured it out. In the future if you think you've run across a bug please post the 3DM model file with your objects in it instead of only a text description or a screenshot.

There isn't really any way for me to give you any information just on a description of something like "union with triangles not working", there are a lot of possible things that could be going on like the triangles are very slightly apart from one another instead of touching, or your objects have some open edges in it instead of being a closed solid, or there are self intersections in some areas, or other things like that.

There is no way for me to figure out things like that from a written description but with the actual model file I can examine the particular situation and possibly give you some proper feedback on what's going on there. Maybe it is a bug, maybe it is something that needs to be tuned up or avoided in the geometry. NURBS modeling tends to be more sensitive to certain kinds of messy geometry like self intersections than polygon modeling.

Thanks,
- Michael