He is a joke. There really isn't anything he can do Darius or Renekton can't do better for the team which is a shame because for a good few years he was always a steady, reliable, easy to pickup champion that never excelled but was fun to play and you could make a difference with him even if you couldn't carry you knew you could impact the flow of the game. But you can't now. If you lane vs Darius as Garen you may as well put your keyboard down and go have a shower because there is literally nothing you can do vs a good Darius - a champion which is currently utterly broken by late game.
Lets break down why Garen is in a horribly poor state shall we:
The lack of tenacity is soul destroying in this current meta of hyper motility champions such as Ekko and yasuo. One single CC and you're done for. Give up the chase or get bursted by say utterly overpowered champions like Ryze.
The Q silence nerf was too much. There was a time AP players would fear that silence now they laugh laugh knowing the effect will wear off before he can do serious damage (because the E tick rate sucks - see below) at which point they burst you or - with the lack of tenacity above - they get away. Ekko, Yasuo, Ahri, LB the list goes on. And these are if you haven't noticed champions that are getting high pick rates right now and it's obvious why.......
The E tick rate is acceptable - just - but I personally feel its too low at lower levels but I wouldn't say it's broken like the above. Just mentioning it because I feel it is warranted.
The Villain mechanic is for the most part not at all helpful to the team. With a snowbally high motility season like season 6 its a moot point dealing true damage to someone when you can be readily CC'd though lack of tenacity and they can just flash / jump / ability away while laughing at you. It does nothing for Garen let alone the team.
Finally Garen's 'passive'. While semi-useful in poking & feeling out fights in Gold & upwards most TF's in silver / bronze are all in and end in seconds until respawn so the passive becomes redundant. If 80% of of the playerbase are silver and below then 80% of the playerbase are not getting any real use of that passive potentially. Worse yet at low levels the passive is so slow it's useless so doesn't offset the lack of tenacity in laning phase or mid game.
Garen overall right now is horrificly broken. I love this champion but I hate his place right now. I don't want to see him buffed to god status because then I can't play him but I also want to play a champion I feel is playing on a balanced and fair system which Garen most certainly is not.
I'm actually saddened that RIOT hasn't addressed this already - at least publicly hat would go some way. With all their data mining and data analysts I think the issues I've raised above would be pretty obvious to anyone involved in balancing the game.