Iirc when you reach t5 with empire mountain pass forts, your mortars are swapped for the hellstorms which have a much flatter trajectory. I tend to pull back from the walls and use that slope as a choke point, with the noble slightly downhill creating a big blob of enemies for the mortars to pummel, this tactic doesn't work with hellstorms, which makes the battles much more difficult.

Another downgrade for me is when your HE capital city garrisons go from t4 to t5, you lose LSG in return for some white lions, which is a downgrade, even when facing high armour opponents in any hands.

Dark elves suffer at certain tiers too (3 I think), when you lose spears for corsairs, you're left with no anti large to hold gates while your darkshards shoot down. Thanks to the latest update though you can recruit a lord and a few spears in 1 turn if you see the attack coming, even with no barracks