One example: in order to pass through the Elemental Plane of Water on the Astral, you need to find the (moving!) portal. In order to do this with optimal safety, you want to have: genocided all sea monsters, so you don't need to worry about krakens and the like have a means of magical breathing, in case you do end up in the water rustproofed all your metal armor and other gear have a large stash of Scrolls of Gold Detection, in a Bag of Holding (itself blessed and stored inside and oilskin sack so that water doesn't get in and erase your scrolls) have a means of safely giving yourself the Confused status (reading scrolls of Gold Detection while Confused lets you detect magic portals) a means of swiftly removing said Confusion, so that you can... *use your Boots of Speed (or other speed-boosting effect) to close rapidly on the moving portal.
And that's ONE challenge on ONE level (although its deep in the endgame and considered one of the harder things in the game). Now imagine doing that for 100+ levels of gameplay... you CAN virtually guarantee a win in Nethack, but its never fast.
I spent a couple years playing hard in college. I read spoilers, and save-scummed for a while, then stopped save-scumming because it wasn't fun and I didn't need to anymore. I've lost track of my legitimate ascensions - at least one with every class, and then I started in on conducts and score maximization.
Now, I still play on and off for a week here or a week there. I can ascend about 1 in 4 times with the easy starting classes like Valkyrie. My YASDs inevitably come from rushing or not thinking things through, plus very occasionally getting screwed early by the RNG.
And I'm not particularly good at the game. Really good players can virtually ascend at will, with the only limitation being their available time.