I think I found the reason for the complete wins of artillery versus cities. Withdrawal chance seems completely messed up. I tried out the mod (never done before, as I only play TAM & FfH - sorry ) just to test some simple cavalry versus cavalry or cavalry versus machine gun. I noticed that my cavalry never retreated from battle. To test out my hypothesis that withdrawal was broken, I edited the Cavalry to have a 100% withdrawal chance. Now the cavalry unit wins every battle, even against vastly superior units. I guess this also explains why artillery with their high withdrawal chance often can decimate entire city defenses.