CustomPotionEffects should be pretty much equivalent to the /effect command:

With /effect, infinite duration saturation, instant health, or instant damage can be applied. Potions can only apply these “instant effects” for a single tic if the duration is set to -1. Other effects which are not instant work correctly

Additionally, /effect can safely apply effects with amplifiers up to 255, while CustomPotionEffects potions can only go up to 127 before the effect behavior is nullified.