In the world of achievements, there are multiple reasons why certain ones can be looked at as an incredible pain, or just straight impossible to unlock.
One of those reasons is luck. Some achievements take RNG into account, where multiple things need to go perfectly your way for you to even have a chance of unlocking the achievement. You could put hours of work in, only for one random factor to ruin everything.
Another aspect is the amount of time it takes. There's achievements out there that take as little as seconds to unlock, like the infamous achievement in The Simpsons Game that unlocks after you hit start on the main menu. Then there are some that take hundreds upon thousands of hours to unlock.
Then of course comes skill. Some achievements are cake-walks, like ones that require you to complete levels in a game's story. Then there are ones that want you to master all aspects of a game to unlock. It's like beating Call of Duty on easy versus completing every boss battle in Cuphead.
Combine all three of these factors into one achievement and you have yourself a nightmare.