I cannot speak on the rest of the series, but I have played devil survivor 1 and 2:
Devil Survivor 1 does have a bit of a difficulty curve that can take one by surprise with the first major boss and it is like priming the player towards what to expect but its story I personally enjoyed.
Devil Survivor 2 is lighter in tone, well compared to the Devil Survivor 1, but I felt it was a smoother experience - doesn’t feel as tightly packaged but it does compensate with having a better presentation and provides choice in a lot clearer manner.
I liked Devil Survivor 1 story better but enjoyed Devil Survivors 2 gameplay more