To be fair, flat is the most annoying terrain on a SS. Hills, you just ride them quickly or pedal harder. Keeps you fit.
Downhill, you learn to coast and carry speed more.
Flat, if you’re going a decent distance or with geared people is a pain, holding very high RPM hurts a lot more than winching up most hill!
SS makes things _simple_ not difficult. Sounds silly, but there’s less mental load. You just pedal. The bike is quieter, the lighter rear wheel improves handling. The inability to just click to an easier gear to get going, or higher gear to crank faster means you have to carry speed more, be smoother, “flow” more. I find it a “purer” experience. It’s also most of the time not the disadvantage you’d think it’d be vs. gears.
There are some maintenance and reliability advantages too, but this is why I have a SS commuter not the MTB. Though there’s a lot to be said for SS on muddy winter night rides.