A large portion of gamers probably encountered the following scenario, they kept grinding a game just to unlock an achievement, even though they were already fed up with the game. That is exactly what achievements are designed for, to motivate you playing in a scenario where you would normally stop, because it became to boring.
Most games now have achivements, often combined with many other bad design choices. I did not really know whether achievements are good or bad, since they can be a good part of the game to motivate you and give you feedback about well your achievements in the game, but they also can be used in a bad way to simply motivate you to do boring tasks you would otherwise never do. So it seems to depend on how they are implemented.
There are two main ways achievements can be implemented in a game:
External achievements have nothing to do with the game itself, they are just tracked outside of the game through the gaming platform you play it on most often provided by the game manufacturer or distribution platform.
So it is mostly to motivate you to get some kind of medal you then can show off to others on a social platform, or maybe you just want to collect them to prove yourself something. Those achievements are in most cases rewarded based on often meaningless or stupid things you do in a game, the game keeps track of all actions you do and if a certain combination of actions is done, it rewards you an achievement.
Internal achievements are part of the game itself and they fit into the context of the game and they are not tracked outside of the game. Those can be in the form of medals or special items you receive in the game. For example you won a race and get rewarded a medal that then shows inside the game in your room or so and you know you achieved something like won a special race. Those achievements are based on meaningful achievements inside the game, they don't reward random or stupid behavior.
You can see that if the context of the game allows for the implementation of an achievement system so that it logically fits into the game it can be fine and justified, but if they are forcefully added externally just to have achievements it may likely be bad.
Why is it bad? An achievement is like a medal, which gets perceived as a physical reward by people subconsciously and so it is like paying people to do certain things like play a game. So people do not play the game, because it is fun or they want to play it, but they are working for an achievement. Sure the result may be positive if you only look at the playtime, but you are not actually increasing the players experience, but in fact make it worse, you just trick him into spending more time than he would normally.
I observed myself playing games a lot and if I care about achievements or not and I noticed in most cases I do not care at all, I just play the game and later look what achievements I got and then leave it at that and do not spend extra effort in getting achievements. However sometimes in rare occasions I become addicted or curious and I try to get all or some of the rare achievements just to later notice how pointless it all was. Recently I spend like 50 hours on a game and gave a lot of extra effort to play through the game very clean, just to get an achievement, just to later notice I did not get it, because I made a mistake somewhere or the game glitched and it did not count and this made me very angry, but then I managed to stop the hunt for an achievement, otherwise I may even have made another playthrough just to get the achievement. You see those psychological tricks can even trick someone who knows how they work. Many people now might argue that they do not care about achievements and most people do not, then I say, that may be correct, but achievements are similar to other bad design choices like adding gambling or pay to win, most players will just ignore it, but a small portion will completely fall for it and will be harmed a lot.
There are even some games that are purely based on getting achievement to make fun of that mechanic and the game itself is a very dumbed down version of a game, but the game is still very addicting. I will not blame them, since they are clearly made as a parody, but still some real games come close to that parody level, but they are serious.
Sadly it is also one of those trends that you hardly can turn back, since having achievements gives you an advantage in player retention vs games that do not have achievements. It is kind of an external psychological cheat code to get people to play something without actually improving the game, this is what makes it so bad in my eyes.
When making a product like a game in this case you have two choices to get people to use it: a) You improve the product or b) You use external tricks to get people to use it. Well and achievements are mostly an external trick, it does not actually improve the game or the players experience.
A legitimate implementation of achievements could look like that certain behaviors that are good gameplay and increase the gameplay experience for the player and other players could be rewarded with achievements which then in turn give you some benefit inside the game like experience points or money t hat then can be spend inside the game to improve your character or something like that. In this case both the game and the experience of all players is improved, but external achievements hardly do anything, you cannot even show them off to other players inside the game. If they would show to other players in game it would motivate them also to play nice and get those achievements, so it benefits everyone.
As you can see I tried to come up with a better implementation of achievements and how they could work to actually do something good, but if you just add them externally just for the sake of having them you don't actually improve the game.
But also internal achievements can be bad, if they are overused. When achievements were initially introduced there were still outcries how bad an implementation of achievements is, but now they are kind of normalized and hardly anyone cares anymore. The initial critique was based on the fact that for example in a multiplayer role play game there are often very grindy and monotonous parts you have to do to gain experience points or money as reward and well the introduction of achievements made this much worse. Normally players would probably get bored quite fast to do monotonous tasks, but with achievements people were much more motivated to do boring things in games, even if they fitted into the game and gave ingame rewards. So a developer should be careful with such mechanics similar to gambling mechanics.
Achievement mechanics can be harmful even, if they have not even been integrated into the game. What does that mean? Well if a game for example has some kind of statistics, but not explicit medals or achievements for them, people can still see the statistic as a motivator to do something just to increase the statistic in that certain part. For example you can collect 50 diamonds in the game and the statistic shows you how many you have for example 28 of 50 collected, so this already may motivate many people to collect them all, even if an achievement does not even exist for collecting all. I remember a long time ago playing against someone and he would always be worse at the game than me, then he played certain boring parts of the game, just so he can be better at something than me and in fact he was, since those things were too boring for me.
Finally you can do a test yourself how bad achievements are and test yourself if a game is still interesting, if you already achieved anything. In many games the motivation drops a lot if you achieved everything, but a good game should motivate by itself and not by external motivators. Beating a game by itself should be achievement enough, you do not need an external medal, you already have the proof inside the game and maybe even already a highscore or you can make screenshots or videos of your achievements, if you want to show off. Achievements are like a parent figure watching you play and giving you rewards for good behavior, while in reality you should be able to figure out by yourself what your achievements are. This article has gotten longer than expected, initially I just wanted to write a short one since I thought this is just a smaller issue, so I think I will stop here, before I get even more ideas, but I think you got the gist why achievements are bad in most cases.