We don't carry Ariat tall zip boots at my tack shop, but regardless I personally have never had a problem with them. I know of a couple people that have, but I have also heard of top-of-the-line zip boots doing the same.
I'm sure other brands have some zipper issues as well, but if you get a chance to look at Ariat zippers they have very very small teeth, which are not strong. Not a great idea if you think about how much force a boot zipper has to endure as you're riding, esp with an ideally snug fit. Personally, I'm sick of Ariat boots but I can't find anything better.