Aside from demos, I recently picked up Hacknet (not Nethack), and I've been enjoying it. It's like a text-based Hypnospace Outlaw, though obviously a different flavor of writing. You do everything by typing in quasi-unix code shortcuts, with minimal clicking. The interface is a bit intimidating. It's very easy to make mistakes, and often quite hard to unmake them. The in-game notes system is also intentionally difficult to access while doing any actual hacking, so it's better to write your own in pen. You also have to focus on it pretty closely, so it's not a game to play while stoned. Unless you do this kind of thing for real, you'll probably need to pay absolute attention to succeed in this game.
As someone who only ever vaguely understands information technology and programming, I'm not sure how real a simulation it is. It seems pretty legitimate compared to the hackneyed 'pipe-dreams' hacking minigame or the kind of thing you see on shows like 'Special Detectives: Miami Lizard Unit.' You could probably have someone on one of those shows just play this, and it would look a lot more convincing than whatever on-screen effect they'd normally superimpose.
The difficulty comes in weird waves, and in unexpected ways. I say waves rather than spikes, because the game is quite difficult as a baseline, at least for me. I find it quite challenging. You're introduced to new concepts in broad terms, and then step by step guided through each significant term and function one small job at a time, with each task offering less and less guidance. Then the training wheels come off, and it's important that you internalize all that information quickly to make it any farther, because now it's all on the clock. Time limits are tight, and you essentially have to learn to to run your operations in the right sequences, with a bare minimum of milliseconds wasted since you can't activate all your hacknet activities at once. Even when they're all properly executed you still have to navigate the breached system(s), locate your target(s), tamper with them, DELETE YOUR ACTIVITY LOGS(!!!), and disconnect before you get got. 90s is a pretty common time constraint in the early game, and I can't imagine being fast enough for the late game. But I can imagine the difficulty will have a lot to do with how practiced a player is at coding in general. For a casual end user-ass scrub like me, it's pretty hard.
The waves hit here and there, in interesting ways. The game introduces new twists to your procedures almost constantly, so you have to be on your toes and hit the curveballs as they're thrown. Some twists will require you to add new rules to your procedure, and others may change rules you've been trying to memorize and optimize, requiring you to revise and re-learn your operation. Active security and counter operation are suddenly there, and you have to remember the procedure for surviving that and implement it quickly. You don't get to practice. You will always have to adapt to new and changing rules. It's almost cruel how willing the game is to throw the player in the deep end and I love it for that.
It all adds up to a more intense feeling - I think the true strength of the game being in this format is the immersive nature of it. With so little information to go on, you start to pick up on smaller and smaller motes of data to build a picture of what's going on. Everybody is basically ghosts, and they feel about as real as anyone you'd interact with on the real internet. A ghostly suggestion of another intelligent being in the same space. A phantom feedback. Which makes it a little spooky feeling when one of them has you in their sights.
Overall it's a good, probably challenging, and high investment sort of thing, and worth the price of admission I think. It's feels harsh, like noodling around on the internet used to feel, when I was a kid, and the internet was weird and untamed. The writing is cryptic, sometimes funny, sometimes not. There's doubtless a lot of easter eggs hidden in there for those in the know. A lot of value for the right kind of player. It makes me feel like maybe I do understand computers a little, which is a nice and rewarding feeling, and worth the tense moments of dread when I'm actually (fake) hacking, for (fake) real!