Citadel, Swamp, and Inferno are, in my opinion, the weakest. Tower is roughly on the same level. And Miriam's skills used to be decent, but now, after giving her the "Logu" skill, they've turned her into an overpowered hero. And besides the starting advantage with the Cove, this overpoweredness applies to other castles as well. I don't understand, Kostya, why you're so fixated on the "Logu" skill for Miriam...
Specifically, it's because of the 6LM (level 6 Large Monster) skill. Without Miriam with the "Logu" skill at the start, I personally don't know how to play the Cove on a HotA (Horn of the Abyss) map. Citadel, Swamp, and Inferno immediately have a strong unit stack. But starting with Cassiopeia or Derek on a 6LM map... well, I don't know...