If you do sprite sheets you need to provide the frame size and padding (if any) then it's bareble to me.
The problem is besides usually having to guess those information, the animation state and amount of frames for each animation also needs to be manually handled, whereas with one frame per file you can simply make a name convention for each animation (like "attack", "hit", "idle"...) and you will be able to fully automatize the sprite loading for your game.
Nevertheless, I understand some engines work quite better loading directly from sprite sheets.