Like any other brand, some of the watches definitely aren't as robust as the others. With that in mind, most people don't wear a watch every day... I wouldn't say the quality is bad, but they are fashion watches. They're certainly better built than a $20 fashion watch though.
I mentioned in the other thread that I wasn't the happiest with the faux horn MK watches because of the acrylic crystals. It's more of a personal preference than anything. I just really prefer a weighty, substantial watch with at least a mineral crystal... I like sapphire best though. I think most people don't know what kind of crystal their watch has nor do they don't care.
I've never had a mineral or sapphire crystal chip or shatter yet, and it's more likely that mine will accidentally end up scratched so I'd rather have something that's as scratch-resistant as possible... or at least more so than acrylic. The crystal on my cream-colored faux horn watch somehow managed to get scratched quite badly. (It's at least 3 years old, by the way.) I also dislike that acrylic crystals can discolor and turn yellow. You can buff out the shallow scratches a reasonable amount though, and acrylic crystals are cheaper to replace. I just don't like the hassle.
There are people that prefer acrylic too...
I wouldn't worry about it. The watch won't fall apart on your wrist for no reason.