In short, it’s better to have a greater part count that is adequately dispersed and spaced out than it is to have a lower part count that is primarily composed of un-optimized meshes or unoptimized meshes.
A lot of people forget what unions are made for originally, they’re to create advanced objects that couldn’t have been made prior with ROBLOX primitives (parts, wedges, spheres, cylinders). There are many usecases for unions, however the usecase you’re defining isn’t going increase the performance of your game.
Additional resources I’ve written in the past that also help answer the question:
Conclusion
Don’t throw computer generated geometry into another software, and throw in back into ROBLOX and expect good results without actually editing the meshes. Good luck, hope this helped!