Ascension in Slay the Spire is more or less the opposite of ascension in Kingdom of Loathing. In KoL, ascension is New Game+, where you start over with more power and options. In Slay the Spire, ascension is a progressive hard mode, where you start over with less power and increased difficulty.
Ascension has the merits in game design. It adds an optional hard mode, which is great for players who have mastered the base game and are looking for increased difficulty. It is progressive, with 15 increments of difficulty that are cumulative. You need not play at the highest difficulty level you have unlocked, although that is the only way to unlock the next one. Ascension progress is not lost on failure; you can try that level again.
The downside is that not all the difficulty increases add more fun. Players tend to like dishing out bigger numbers, but they rarely seem excited about being hit with them. Six of the fifteen ascension levels are increasing numbers (damage, health) on the enemies (regular, elite, boss), plus shifting the odds of negative outcomes on events. Three of the difficulty changes are lowering your health (start damaged, heal less after bosses, lower max health). Three are reducing your resources (less gold, fewer upgrades, weaker potions). The other two are adding a curse (dead card) to your deck and increasing the number of elites.
This last seems the most interesting. Increasing elites is usually a good thing. Better players with stronger decks seek out elites, because beating one gives you a relic, which then gives you more chances for synergy and higher power. If your deck cannot beat elites consistently, it is going to have real trouble beating the boss. I am not yet to the highest levels of ascension, but I am led to believe the pendulum swings the other way later, when the increased damage from number-boosted everything means you cannot afford to fight as many elites as in the base game.
That also becomes a point where increasing numbers cross a threshold and do something more interesting than just increased numbers. You want more elites because you want more relics, so power yields more power. Once you cannot spare the hit points for the elites, that synergy goes in the other direction, less power yields even less power. That is an interesting and elegant outcome from a straightforward shift, although it seems like a lot of tweaking of numbers to get there.
I am gradually making my way through ascension mode on the two available characters because it is the progress and “something new” available right now, other than the daily challenge. As I am getting into “we take away your resource” levels, I am not really having more fun. Sometimes making decisions under increased restraints is fun because of the intellectual puzzle involved, but sometimes that just restricts the range of options to the few strongest, which narrows the game rather than adding anything new.
The daily challenge mode provides a window into how ascension could be handled differently over time. By mixing in some negative modifiers from that mode, ascension runs could be different instead of just having different numbers.