some champions can not play vs other champions unless they get terribly fed/reach end game etc. that said Garen isn't particularly strong vs a set amount of champions. he is however generically good at what he does but lacks a hard gap closer which is almost a prerequisite of a top in the current meta today.
I do not see how Garen is any easier to play than any other top champion tho. they all pretty much lack skillshots and exotic manipulation of the environment in your favour.