Yeah for the current difficulty we leaned towards the side of being easier rather than too hard, so that players will be able to learn the mechanics. We'll address that in a balancing update.
Thanks for the feedback regarding the specific units. Brawlers upgrade may be abit overtuned but if we introduce more stronger singular enemies it may balance itself out.
For cannon leader and flint sparker, since they're 3 cost we wanted to push their power a little bit. Since having to defend multiple lanes, spending all 3 energy on 1 Lane may be difficult. This will become more obvious when we introduce stronger enemies that come in quicker, piling on the pressure much sooner.
Rarity is something we're already planning to implement, but have not done it yet due to the limited pool of cards at the moment.
Thanks for playing and leaving feedback!