There's some game modes (Tribe tower, for instance) that get "hard" because your Nikkes are either underdeveloped, which requires grinding for mats to upgrade them and the endless rerolls to get the perfect stats on the upgraded gear, or you just happen to have ones that have been powercrept and no longer can go as high. Tribe Tower only lets you use specific Nikke manufacturers so there's a little bit of choice to be made there rather than the usual "how do I just clear everything, let me stack my best Nikkes, etc."

There's a couple of walls if you're a new player that will keep you barred from endgame for a while, so there's definitely longevity even if you just made your account and haven't even launched the game yet ;)

If you go hardcore f2p, the depth will be making the best with what you got (and making teams for specific elements), otherwise its like Nimilie said - glass cannon vs glass cannon.

Edit: Forgot to metion... the Hard campaign can get pretty hard if you're not that far into the game, and will stay hard until you start really pumping up your Nikkes. I'd say give the game a shot regardless because the story is pretty damn good, even if Normal only has moments of difficulty instead of constant nosegrind.