I've created a couple of different digital assets. One of them works fine in unity, but the other is either invisible, or just has no geometry.
They both look and work fine inside of Houdini, and I can't find any notable differences in the settings between them.
Does anyone know what could cause this sort of error?

Check the Console window in Unity for any warnings or errors on import. Dive down into the hierarchy of the generated gameobjects to see whether the mesh components were set properly, and the materials/shaders were assigned. Check to see if alpha is 0 if they do have materials. If mesh looks valid, and materials have been assigned, could be that it might have vertex colours with 0 alpha, in which case change the shader on it to one of the Standard shaders and you might see it as those shaders don't use vertex colour.

If all else fails, please attach an HDA with the issue that I can debug.

Well, I'm… Not sure what happened. I opened up the project today to test out your advice, and it was just… Working.

Could have been just a graphical bug. Sometimes it's also good to test with another version of Unity.