Exactly, so if you run pacman -Sy something and that something has an important dependency that has also been updated, e.g. glibc, you have now updated glibc without updating all of the packages that depend on it, causing them to stop working. E.g. pacman depends on glibc, so doing pacman -Sy something when a new glibc version was released since last you updated essentially breaks your install.

This is because Arch assumes you know what you're doing, and let's you do it. Great for power users, dangerous for people who don't know enough. All of the things I mentioned are similar things that look innocuous, but will cause you huge headaches if you don't know what you're doing. And they don't happen on other distros, because the other distros purposefully try to stop you from doing them.

Edit: BTW, the speed in which you discovered what -Sy does tells me you're far from an average noob user, for you Arch will likely be a breeze, but you need to understand that the average new user will not read manuals nor the wiki, when you recommend stuff to people wanting to migrate assume it's your grandma, you wouldn't expect her to read through technical manuals to use her system.