Mesh Boolean Union is the component that would do what you want. In cases where the mesh is very shattered, as you noticed, Rhino’s current algorithm might not give good results. Improving this type of operation is something incremental, as there are almost infinite possibilities of combinations of faces.
There’s a plan for improving the algorithm, but it’s not ready yet. Here the current schedule: Mesh Intersection Milestones in Rhino 7
If you have cases that you feel should work and want to submit them, they can be added to future tests and can be checked while writing the algorithm. Please submit them if you can. You can simply attach a sample here.
Also, to get a more-easily working Boolean union (the principle is different, more robust but slightly less precise), a way is to use implicit surfaces/metaballs/marching cubes. These names are essentially pointing to the same concept. You could check out this Grasshopper discussion or JellyFish.
Thanks,
Giulio
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Giulio Piacentino
for Robert McNeel & Associates
giulio@mcneel.com