For me I think his passive and health are issues. Garen was never about heavy damage except the old IE build which was fun.
People say Garen is tanky but he isn't. I remember the days when he was but now Sion,Chogath and Ornn are so much tankier. And his resistances need to be built up via CS farming. Darius is even tankier in some way he has more health and gains more per level. And more AD.
Leading from. This Garens passive is almost non existent before 11, his mini game seems to be survive till 11 then you're ok but try doing that vs a competent Darius whose entire lane phase is denying you farm, shutting down your passive by using chain cc to lock you into combat whilst healing himself.
A good Darius will simply lane freeze and cc the hell out of you when you try and farm and if you're unluckier still his jungler will be sitting in a bush just waiting for that moment. Free farm for Darius and a starved Garen. Not fun at all. A passive change would improve this sort of bullying massively and make a tremendous quality of life difference