Nikke's (at least imo) biggest pros are its awesome BGM, the graphics and game play are commendable too, there's no energy/stamina system, and lastly you can play on your own pace.
Since again, energy/stamina is not a thing in Nikke. But like almost every gacha game, your game progress in the campaign or in any stages of the games' content will be stalled eventually the more you progress. It's a great side game, F2P friendly, and fun. The game play is quite unique and there are things that might confuse newbie players, especially when building their Nikkes, team compositions, and strategy, but hey, theres YouTube so, don't worry too much about it.
And did I already talked about the waifus? No? Well, better find it for yourself then. Ehe :p