The original Autoprime fed primers from a round tray and there wasn't a cut-off to prevent having the next primer sitting immediately in adjacent contact with the one you are setting. I'm sure Lee was concerned about the possibility of setting off the entire tray. Their recommendations changed over the years. When I bought my first (old-style) Autoprime, they said to use ONLY Winchester or CCI, and to never put more than 20 primers in the tray at once. A couple/few years later I bought another, so I could leave it set up for small pistol, and the recommendation had changed. They still listed CCI as okay, but Winchester was out and had been replaced by some other brand. (Don't remember whose.)
I've done tens of thousands of Winchester large pistol primers with my old-style Autoprime, and several thousand Winchester small pistol primers with the other old-style Autoprime. I do adhere religiously to not putting more than 20 in the tray at once, and I wear eye and ear protection when priming, but so far I haven't had a problem.
The new Autoprime, which I don't own, apparently incorporates some kind of cut-off so there's not supposed to be another primer sitting next to the one you're setting. Since I've never seen one of the new ones up close and personal, I don't know just how that functions.
One of the selling points for the new one is that you can now drop a full 100 primers onto the rectangular tray at once. I have seen the new tray, and I don't like it. Once the primers have been deposited, the idea is to let them all slide down to the half of the tray closer to the priming tool, then you flip over the other half of the tray and lock it closed. But the "hinge" isn't a true hinge, it's what is laughingly referred to as a "live" hinge -- which means you're just folding the plastic. You know that's only good for 'X' number of cycles before it breaks.