Binary, trash champion that with a elementary kit that can go insanely tanky but deal great damage. No mana. No resources. Can just fucking split all day long with no repercussions. Can melt ADCs and mids even if he goes full tanky.
Not fun to play and not fun to play against. Delete this fucking abomination of a champion.
There's nothing wrong with a champion being simple. It's a double edged blade; you don't have to think too hard to play, but your opponent doesn't have to think too hard to understand exactly what you're able to do. Some people like this, some don't, but there's nothing wrong with this sort of design.
There's nothing wrong with being tanky with damage. That's a major part of all Juggernauts, and they pay for having great defensive and offensive capability elsewhere. It's not like Garen can do everything, he has major weaknesses that are easily exploited.
There are a number of no resource champions in the game. So what?
Any champion can split push all day if no one understands how to properly punish them for it. Take some time to develop a plan of action when someone is pushing like that and you should be fine.
The reason Juggernauts are allowed to melt squishy characters is because those characters typically have the ability to kite, self peel, or should be guarded by other members of their team that can assist with peeling. If you are out of position as a squishy character you should not be surprised if you get punished for it.
Fun is relative, and not everyone can be pleased. This is a fact of life you're going to have to learn how to deal with sooner or later, and I highly recommend you pick sooner.
Alright, once and for all, let me explain.
Garen is a type of champion known as a Juggernaut. Juggernauts are a subclass of the Fighter group of champions; Juggernauts specialize in dealing and tanking large amounts of damage. The damage comes from high base stats, the damage-focused abilities in their kits and usually the purchase of 1 to 2 attack-related items; the resistance comes from innate tankiness via high health pools, sustain through regeneration or conditional heals in their kits, and can be boosted by the purchase of Armor, Health and MR items. This high damage output AND damage resistance comes at the drawback of being a relatively slow champion that can have a hard time sticking to a target if said target is more mobile than them.
The fact that Garen, or any other Juggernaut can kill ADCs and Mid-laners (Whether Assassins or Mages) or any other champion, just means he is DOING HIS JOB AS A JUGGERNAUT. Being a slow, but powerful melee (Or in the case of 
, limited ranged) carry once they can get their traction going, like the powers of his subclass namesake, the Juggernaut of Marvel/X-Men Comics. I fully realize that the word or term "Juggernaut" did not originate with the character, but in terms of a NON-League representative, he does a good job epitomizing what the Juggernaut subclass in LOL is intended to exude in terms of thematic.
In terms of your remarks on an "elementary kit", Garen is designed to a simple, pick-up-and-play style champion. A beginner-level champion, but he has the potential for more intricate play once you get the basics down. His builds can be quite varied for example, but all have the potential for success if you play it right.
