yeah, slay the spire has ups and downs in difficulty. I'm not 100% certain its current balance.
but, last I played. it was beatable at first. However, actually got harder with your first few unlocks. as RNG goes up, and its harder to make a coherent build. new items, but not new builds.
as you keep going though, the RNG get bigger... but, you also then get more good then bad cards/relics. and can pick and choose. and those new cards/relics are actually forming new builds, for more options.
much of it is player skill then too. the rng goes up, but you know what to look for and how to pick good cards, and not take too many, etc.