After playing for around 20 hours, here are my thoughts.
It's a really solid concept and is already loads of fun, despite the limited content. I will absolutely be following this game and hoping for a full release.
There were a few things that I found somewhat off-putting, however.
Most notably is that the game reaches a point where resolving all the attacks of a single card played takes... a while. There are 50+ enemies per battle and in one go you can clear out at most 5 of them. That means a minimum of 10 waves. Playing a card takes a second or two, but waiting for it to resolve can take a minute. That's way too long to just sit and wait.
The second thing I noticed was the lack of single target damage. Not that it doesn't exist, however, in all of my runs my characters would end up dealing more damage to enemies other than the primary target.