Gameplay-wise, frankly, FFRK is boring. The UI, while better than in the past, could still use some improvements, the game runs very badly on old smartphones, there has been the whole "DeNa changes the drop rates of the FFT banner" thing, and the only interesting bits of the game are the boss fights (and even then, it is mostly 50% preparations, 40% S/L and 10% actual playing time). When new events are released, my first reaction is "Ugh, I have to waste my time auto-battling those stages".
DeNa did a great job when it comes to sprites and abilities/Soul Breaks animations. But the rest? Simply boring. I was quite invested for a few months, but I play this game less and less, and I don't think I'll miss it once I definitly stop.