The difficulty of each achievement varies some achievements are very easy (e.g. It is possible to see the achievements sorted by the percentage of users that managed to complete each achievement by going to the game's global achievements at Steam.
Achievements have no in-game effects the achievement just pops up in-game, and is then added to the player's Steam or Xbox profile depending on the version of the game. Hearts of Iron IV has 132 different achievements, all using the Steam achievements mechanics. If you spot a mistake then you are welcome to fix it.