Growth Curves

The growth curves don't affect grailing because stats grow linearly with grails independent of the curve.

The curves mainly dictate which levels improve your servant's stats the most. For instance, Hans has a strong s-shaped leveling curve, which means the higher levels won't change his stats a lot. In fact between lvl 55 and lvl 65 his hp stat grows only by about 300, which means it doesn't make much sense to invest in him beyond lvl 55.

The reverse-s on the other hand means that the higher levels are most important for the servant's stats. E.g., Cu Alter gains only 500 attack between lvl 50 and 60, but almost 2000 between lvl 80 and 90.

I don't think there's "good" or "bad" curves. I personally like the s-curves a bit more because the servants become stronger at lower levels and are therefore available earlier to use for harder content.