People are exploiting Act 1 to farm 100+ mercenary cards, then claiming that the game is "too easy".
Maybe put a hard cap of 10-15 cards per type in Act 1 - especially for Mercenary cards. The rest can be the victim of "Some of our mercenaries (are about to/have quit), Commander!" or some such.
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This is probably not going to happen, but one thing that might have been interesting for Act 2 would be for the different siblings to focus on different units. Farhan could focus motly on air units with some ground support, Sybille could focus on heavy ground units, and Karhan could focus on naval units.
At the moment, they all pretty much have the same stuff. It's only that Karhan starts with a large navy that makes much of a difference. That and he tends to pile units on his territory instead of shipping them elsewhere.
The other problem to that of course is how close Karhan starts to the player capital. It might have worked better for his capital to be in the southeast, so he could spread ships through the seas from there. His current location and the poor ocean tile configuration around his capital means he focuses the majority of his attention on the player.