A power stretcher gives you a tighter, more even stretch in any space if you know how to use it right. The trick is to use short, controlled strokes instead of trying to make big swings. It takes more skill, but the result is a better install that lasts. Calling the knee kicker the only way just shows a lack of practice with the bigger tool.
Wait, you were using a power stretcher for closets? That sounds like a nightmare. Trying to swing that big thing around in a tiny space, no wonder your back was killing you. The knee kicker is the only way to get into those corners without wrecking yourself. I'm honestly shocked you lasted that long without one.