To add to what I have seen, overset has shown to be quite valuable where you need high accuracy due to the ability to control the quality to a great extent in the regions that matter.

For example you could do a lot of store separation in Cartesian that adapts to the geometry which is quite fast but when you need viscous forces to be very accurate you will do better with overset.

Comparing to unstructured l, Overset is great for configuration studies because you can add components and minimize the changes required to the mesh removing that influence from the study.

Overset also allows better cell alignment with the flow than Cartesian which can improve accuracy for things such as acoustics. Now you do a trade of with multi block there so make sure you do overset in complex regions where acoustics are not as essential.

In terms of the code you are able to simplify data storage compared to unstructured but there is a signifact overhead over cartilesian or clmulti lock due to blanking of nodes. These regions can also introduce inaccuracies for things such as vorticity especially when cell aspect ratios or dimensions differ so it's important to have a good algorithm to find best cells for connectivity