You mentioned that units have a set of stats that never change. Is there any type of advancement (automatic RPG-style, new equipment, heart container collection, buying upgrades with gold/credits/cash, etc.)?
Does initiative effect both the order units go in and how often a unit acts?
Making the faster units do less damage and the slower units do more damage seems to be a fairly common balancing trick.
Multi-attacks combined with being able to attack twice in a turn may need some consideration.