Hi Matt, thanks, I'm glad you like MoI!

The reason for the union failure is that the cylinders inside the wheels are floating just a little bit in front of the back plane of the main wheel part, they aren't touching - here is a zoomed-in view from the top of one of those wheels:



You can see the dashed lines for the edge of the cylinder and the back plane, there is a bit of space in between them, the cylinder is floating in front of the back face a bit.

To make the union work, these objects must be touching each other - and it is usually not a bad idea to make them actually stick through each other a bit instead of only exactly touch, but they must at least be touching.

It also looks like there is a bit of a slant to the cylinder piece's face - maybe this was done as an extrusion of a very slightly off-axis circle? It might be a good idea to draw a new cylinder in there to get a really regular end on it.

It looks like you're having some fun creating stuff!

- Michael