I have experience with both systems. Unless you are specifically looking for a feature that only a* provides, there is no reason to switch. Or if you are trying to pathfind many many gameobjects (like 50 plus at one time).
One thing a* provided in the past, was a way to regenerate the navmesh at run time. Now you can do this with the experimental features of unity navmesh as well.
A* can have better performance, as it has several types of pathfinding, but it is much more work to setup, and IMO, that performance only pays off, if you have many objects moving at once.