1. shell elements are very good for bending. To get comparable results with volume elements, you need about the same number of elements in thickness direction as the number of the internal shell layers.

2. for in-plane shear, shells are comparable to volume elements.

3. for out of plane shear (X-Z, Y-Z in the shell local coordinates), shells are not good.

Therefore, if punching shear is your main interest, I would definitely NOT recommend using shells in the regions near the column(s) (or where the most of the shear punching happens).

I'm studying punching shear in flat slabs. I used solid elements and shell elements. When I compared the two results there were  big differences, I want to understand why and how can I get a good agreement between the two results?

Regards