Idling is okay. Each unit has 2 different craftable upgrades, of which you can choose 1 to equip. For example Water (your first unit) has Rocks upgrade which increases it's base damage, and Generator upgrade, which increases damage of all other units at the cost of dealing no damage itself, so later on you'll probably have different builds because of this.
The main gimmick of this game however is it's roguelike Dungeons Mode, where with each killed enemy you get resources to progress in idling. And sadly, it's poorly made. There are no sound effects for anything in Dungeons Mode, and since there's only 1 music track in this game, you'll either have to play in complete silence, or listen to the same piano music over and over again. I do have to say that it looks and plays really nice, movement feels nice, enemies are recognizable, a lot of weapons just feel cool. It's not all bad for sure.
The Dungeons Mode itself can be fun, but only when you actually manage to find a weapon. You see, you basically start with a magic pistol, a pretty crappy weapon. Doesn't shoot too fast, small damage, weird spread and just isn't fun to use. You can find weapons in chests, that spawn randomly anywhere on the map, but you're not guaranteed to get a weapon, you also might get a spell or a movement ability, both of which are no exaggeration - completely useless. And what's more is that you might not find any chests at all, and have to deal with easy, but still time consuming (because of your low damage) enemies in hopes of finally finding that chest.
Sadly, after getting a weapon (most of them are actually pretty cool and fun to use) you'll quickly realise that there's nothing else to look forward to, since there are no relics or artifacts that would change your gameplay through passive or active bonuses. After you get a weapon, the only thing left is a marathon of 100 levels. Sure, every 10 levels you reach a new area with new monsters and visuals, but that's it. There are only 3 monsters in each area, and 1 unique boss on every tenth level, so after a bit of playing you'll learn what every boss and monster does, which will make this fairly easy game even easier.
Crafting items for your units is also pretty weird. Every time you craft an item you play a rhythm minigame, and i'm pretty sure that if you do well in it, it will improve item quality. But even though i'm pretty decent at these kind of rhythm games, i wasn't able to consistantly hit perfect in crafting, which makes me wonder whether a person that never played rhythm games would be able to do this.
Everything else is either fairly solid, or i just haven't got there yet since i've only played this game for 2 hours at the moment of writing this review, but those are the things that i've noticed while playing. I still think that you should give this game a try if you like idle games and fine with roguelikes, since i can see how much better this game would be with a bit of polish. And honestly even right now it's still miles ahead of many idle games (which shows how ♥♥♥♥♥♥ many of them are).