Immune seems boring as it severely reduces card interactions and "battle feel". Instead of immunity, I would like to suggest to give units armor, resilience (damage reduction) or slow automatic regeneration if damaged, so that you can at least damage/interact more with them, which makes it more interesting.
In case of Eredin's leader ability to give Immune, this is broken imo. There are cards out there that are powerful engines by themselves and Eredin can ensure that any of these cards is almost untouchable, let alone removable. The Yennefer: Conjurer - Eredin combo played early last round seems to become popular right now. Not much fun and insta forfeit if you're not running crazy boosts, especially with most cards being of similar strength. Instead of Immune, Eredin can give units armor or resilience or something. Frost armor such as in Witcher 3 would be cool
In case of Eredin's leader ability to give Immune, this is broken imo. There are cards out there that are powerful engines by themselves and Eredin can ensure that any of these cards is almost untouchable, let alone removable. The Yennefer: Conjurer - Eredin combo played early last round seems to become popular right now. Not much fun and insta forfeit if you're not running crazy boosts, especially with most cards being of similar strength. Instead of Immune, Eredin can give units armor or resilience or something. Frost armor such as in Witcher 3 would be cool