All in all this is a really satisfying and well put together card battler roguelike. The base game has enough going for it already, and the expansion really solidifiies it as a classic. 10/10 would run train.
The game is amusing for a while. Once you learn the mechanics winning becomes easy. Unlocks are minor and it just implements slightly more difficulty each time you beat it.
After more than 700 hours of Slay the Spyre I was gifted with this Monster Train and so far it seems to be a lot of hours again. The variety of strategys, characters and factions make it very interesting and enjoyable.