Ah yes, someone else with several hundreds of hours in a a game who somehow dislikes it. Hey, at least I know what I'm talking about.
Most of the issues I have with Idle Slayer are not apparent when starting out. And now I also want to 100% it, out of spite.
The early game is actually a lot of fun. The progression is steady (for this type of game) and the mechanics provide a very positive gaming experience. Unfortunately this changes drastically.

There are several very powerful rewards you simply cannot get while being idle. If their rate of acquisition was just slower, that would be fine. But no, they are impossible to get without some activities. The rewards I am referring to are the pieces of gear you use in your Armory. Out of the 9 slots, for the regular (read: non-Key Items) pieces that fit into them, 6 are in some way acquired through playing three separate minigames and fighting two different bosses. These bosses as well as two of those minigames would fit much better in a sidescrolling plattformer than in something called Idle Slayer. I cannot stress enough how much this annoys me. Many people will probably think 'git gud' now, but I do not start an idle game to git gud at this type of activity. If that was my jam I would be playing Hollow Knight.
A small number of the permanent crafting upgrades also require to do the Ascending Heights minigame, which I especially hate, to get the Frozen Shards incredients. It is probably possible to ignore them, as well as at least some of the Armory pieces walled of behind their respective minigame. But why should this type of decision even be made? Either suffer through something which should not be in this genre of game and which I dislike or unnecessarily slow down my progress.

For the second time now, the developer felt the need to do a limited time event. Anybody who starts with Idle Slayer in roughly a month will have missed both events. Anybody who starts now will have missed the first one. Awesome.

There is an upgrade called Inner Power, which has been a problem in the game for a long time now. First it was just too powerful to the point where other activities than farming your minions were mostly a waste of time. Then a huge change/nerf came months back. Inner Power was still too powerful until it became pretty much useless after a certain point. That was also not ideal.
Another big change was made with the current 6.0.0 patch. And now Inner Power is even more overpowered during the time it is useful but gets completely gutted way earlier than before. The worst of both worlds. Galaxy brain gaming design.
I don't think it is impossible to tone down the power of Inner erm... Power but have it be something that helps progression all the way through instead of dominating it for a while and then become useless. But that does not seem to be the goal here.
A couple of other things were introduced that help alleviate the loss of Inner Power in the endgame. This does not change the fact that the way this upgrade was handled is beyond stupid.

Ah yes, someone else with several hundreds of hours in a a game who somehow dislikes it. Hey, at least I know what I'm talking about. Most of the issues I have with Idle Slayer are not apparent when starting out. And now I also want to 100% it, out of spite. The early game is actually a lot of fun. The progression is steady (for this type of game) and the mechanics provide a very positive gaming experience. Unfortunately this changes drastically. There are several very powerful rewards you simply cannot get while being idle. If their rate of acquisition was just slower, that would be fine. But no, they are impossible to get without some activities. The rewards I am referring to are the pieces of gear you use in your Armory. Out of the 9 slots, for the regular (read: non-Key Items) pieces that fit into them, 6 are in some way acquired through playing three separate minigames and fighting two different bosses. These bosses as well as two of those minigames would fit much better in a sidescrolling plattformer than in something called Idle Slayer. I cannot stress enough how much this annoys me. Many people will probably think 'git gud' now, but I do not start an idle game to git gud at this type of activity. If that was my jam I would be playing Hollow Knight. A small number of the permanent crafting upgrades also require to do the Ascending Heights minigame, which I especially hate, to get the Frozen Shards incredients. It is probably possible to ignore them, as well as at least some of the Armory pieces walled of behind their respective minigame. But why should this type of decision even be made? Either suffer through something which should not be in this genre of game and which I dislike or unnecessarily slow down my progress. For the second time now, the developer felt the need to do a limited time event. Anybody who starts with Idle Slayer in roughly a month will have missed both events. Anybody who starts now will have missed the first one. Awesome. There is an upgrade called Inner Power, which has been a problem in the game for a long time now. First it was just too powerful to the point where other activities than farming your minions were mostly a waste of time. Then a huge change/nerf came months back. Inner Power was still too powerful until it became pretty much useless after a certain point. That was also not ideal. Another big change was made with the current 6.0.0 patch. And now Inner Power is even more overpowered during the time it is useful but gets completely gutted way earlier than before. The worst of both worlds. Galaxy brain gaming design. I don't think it is impossible to tone down the power of Inner erm... Power but have it be something that helps progression all the way through instead of dominating it for a while and then become useless. But that does not seem to be the goal here. A couple of other things were introduced that help alleviate the loss of Inner Power in the endgame. This does not change the fact that the way this upgrade was handled is beyond stupid.