A lot of buggy controls.
1. At first, the main issue seems to be HS on crack as your cards can be upgraded to be more powerful. However, it got stuck when building the deck.
For example, you can merge two duplicate cards together in order to get a more powerful card. However, if you have one weaker version and one stronger version, you can only put one in the deck, not both.
2. Deckbuilding is a joke. You started with one starter deck. When you build a new deck, you cannot play it until you click 'Activate', but you cannot un-activate your starter deck, making you unable to play with a new deck. You also cannot delete deck(s) you do not wish to play.
These two issues alone making the fun aspect of playing TCG/CCG disappear. Not talking about smaller issues yet, such as very small card texts, card effects running without a log or explanation about what happened, confusing stories, overpowered opponents which you don't know of until you fight them, areas which you can hover your mouse over but unclickable, etc.
As a TCG/CCG player for 10+ years, I can definitely say that this isn't worth downloading.