Having them impossible to dodge without a power-up wouldn't be that much of an issue if there wasn't also no way to anticipate/avoid them. Imagine playing the game for the first time and suddenly there's an enemy that is absolutely impossible to dodge because you haven't found the right item, and you're forced to take damage. To a player playing your game for the first time, that's a profoundly unfun moment, and is likely to leave a very sour first impression if your game.