I enjoyed Monster Slayers enough to beat it with all 12 classes, so that is an endorsement.
The early game is bad. Monster Slayers has an upgrade grind, and you will not beat the game without spending some time on it. How long is a bit of a question, since because you will be learning the game at the same time, so it is unclear how much of that is “I am failing because I do not know the game yet” and “I am failing because I do not get all my abilities until I fail X times.” (There is also better equipment you can buy in later zones but not equip until you die and start over.) My early game was probably extended because I started with the Cleric.
Start with the Rogue. The Rogue is the easy mode class. I never finished the Rogue’s upgrades because you don’t need them all. Half the cards are damage + draw, so you just dump all your cards on the enemy and watch them die. There is some nuance, but the Rogue is so much more powerful than the other classes that a card called “Enough!” was added to the game to stop those long chains of cards. The Rogue is still this strong after that.
Within each run, the early game is bad. That is, it is trivial. If you are strong enough to beat the game, the first half of each run is a speed bump. You can do better or worse, but you face no real risk. If you have any real chance of losing in the first two zones, you have no real chance of beating the third.
The classes are fun. The differences between them are smaller than “class” would imply, but each has some unique cards that give it a different flavor. For the basic classes, those are clearly labeled. The advanced classes have some fun quirks, even if “quirky” does not necessarily amount to “Rogue strong.” The Assassin is the advanced Rogue, and its trick is putting traps in the enemy’s deck and cycling cards to help them come up. It is fun to damage the enemy with its own deck, which also becomes card denial. The Beastmaster is a Ranger who brings a pet instead of companions. The Apothecary gets to pick potions on the fly. The Necromancer manipulates discard piles and summons “minion” attack cards to bulk up the discard.
As I mentioned, I really wanted the Cleric’s healing plus DOT combination to be great, but either it is not stellar or I really didn’t know what I was doing even when I beat the Harbinger. Maybe I should try another Cleric run sometime, see how it goes. Still, better than the Barbarian and the Brute. Maybe it is just my playstyle, but those seem like the worst classes. Damaging yourself to hurt your enemy is great for NPCs who will live 30 seconds. It is not great for a player whose health does not refill between fights. For the Brute, sacrificing all defense for a crit bonus is not great.
You do not really know a class until you see what it can do on the third map. The top tier cards can change everything, and I say that without even having tried them all. A “meh” class can suddenly turn the corner and become devastating with a card or two. If the late cards are weak, so is the class, because the late enemies are strong. If the good late cards do not come up, you might not be winning that run. Sorry, it’s a roguelike deckbuilder, and sometimes the right cards are never available. Most classes have a late game doubler cards, and maximizing that gives you ridiculous turns that let you beat the end boss quickly. Cast your best spell twice, take another turn, then do it again? Great! Brew an invulnerability potion then copy it four times so you just cannot be hurt? Great!
If the first map gives you a chance to see what your class cards do, the third map lets you see what you really can do.
A downside is that much of the game is spent purging your initial cards. Every class has upgrades to trade out some of your starter cards for your class cards. Like in many deckbuilders, you invest a lot in getting rid of cards, thinning your deck so you do not draw Attack I. Once you delete 5-10 cards, you have just the ones you want, and that is a good deck to play with.
The great joy in these games is getting your engine going, then unleashing it on some hapless monster. Or better yet, some monster you know to be really dangerous, but that doesn’t matter when you can smash it in two turns. Maybe that is the merit of the early game when you are too weak to win: it feels so much better when you are too strong to lose.