PLEASE do not use Unions as a crutch to fix z-fighting! This will cause all kinds of performance issues later on and the alternatives - offset a part by 0.01, or resizing it so it’s not overlapping, or many other better solutions - are way less destructive to the objects you’re fixing for the majority of cases.
Unions should only be used for complex geometry if you don’t have the skill set for other (better) 3D modelling software. There are very few edge cases where unions could be used to fix visual issues, but generally it’s best to reserve them for complex geometry.