If you group parts together to create a model, it effectively does nothing performance-wise and instead contributes to organization/keeping specific assets “together” (eg, you make a table, you group the parts together, and now you can freely move the table around as if it were one object.)
But unioning is completely different. Although unioning “puts together” several parts together, like a model does, it does this by combining the parts together to form a “new part.” By doing this, it creates an entirely new hitbox, new properties, and a higher triangle count. It effectively changes everything.
You can read this tutorial for more on that. But models/unions are totally different.