I don't even care about "hard/ difficult", I just want things to have replay value because they're still engaging after the first few times.
And the big problem preventing that is the extreme predictability of everything and the big gaps between two big brain mechanics happening. Between them happening you have... nothing. Smileton, Eric and Tapeworm Lady are good examples of this but there's plenty more. You have long gaps of nothing happening, where you just stand still and do your thing. This is especially annoying on tank and healer as the former doesn't have anything to mitigate and the latter nothing to heal. But even on several dps it just gets so repetetive. You have long periods of dummy fight, a short big brain mechanic that makes first timers go "ooooh! aaaahhhh!" and then back to dummy fight.

Practice, getting better at your class and game and all wouldn't be so bad if the focus wasn't so heavily on having that 1-2 big brain mechanics but then nothing else because can't overload poor brains, so better not add anything else.
Instead of making it overall smoother and giving you more smaller things to react to in shorter intervals. That's what often keeps content in other games engaging even for experienced players that ran it many times.
It's a matter of balancing the impact of mechanics (how detrimental is it if you fail?) and the frequency and the predictability so you have a more consistent engagement that doesn't suddenly poof after a couple of runs. And you always have binary positioning. There's a safe spot that's always 100% safe, you either stood in it or not. And since patterns are mostly static and repeat, that leads to being able to position in a safe spot that will cover the next 2-3 mechanics and dozing off for the next 4min because you don't need to anything else.

Things like getting knocked up for a bit or a 2s stun or a short slow or a short damage down when failing something makes it low impact but still gives you a clear incentive to try to avoid it. If you fail, it's alright. Not the end of the world, it's more of a nuisance than a real danger. But you have more to react to instead of having a handful mechanics over 10min that look really complicated and like you need to study for it but ultimately come down to standing in a specific spot and you will eventually know before it happens where to stand. That is how things become stale for experienced players: it's a lot of smoke and mirrors but very little consistent substance.
You can add these mechanics and show creativity, not saying they're bad in general. But when the entire encounter is focussed around them it can get problematic; quality isn't always better than quantity and the lack of quanitity makes it stale for veteran players.