I wish there were more cards.
I have played it a decent amount, but I probably wouldn't still play it if it wasn't also on my iPhone (there's a "plus" on Apple Arcade that looks identical, too).
I like Monster Train better mechanically for the reason that it does feel like there's a lot more variety, though I dislike how short the runs are to build a deck with. (I'd like Slay the Spire to go longer on a good run, too).
I haven't been too far on ascensions. I don't think they're really more entertaining. I mostly do the daily runs because at least there's variety there.