The attack on Gerbent was postponed until the artillery had some more power. As it turned out, even 6 nearly fully powered artillery regiments barely removed 30 entrenchment points out of the 140 or so. Artillery is truly underwhelming.
As Gerbent functions as a port, the AI also doesn't face supply problems. Still, even units that are out of supply quickly entrench again in their part of the turn after being attacked. Few things are more annoying than some out of supply units entrenching back to 50 over and over whilst your artillery is more or less powerless.
There's another issue: The manual implies that panicking units retreat. They don't. As long as a unit in the hex doesn't retreat, the panicking defenders also stay where they are. That means, for example, that when you encircle a stack, as long as 1 of the units in the stack doesn't retreat, the others will remain in the hex unless they break or retreat normally (although I don't think I've seen many retreat results for pocketed units).
WitE Alpha tester
Panzer Corps Beta tester
Unity of Command scenario designer