The standard for class auto-loading was created to go hand-in-hand with the then new package manager Composer. PSRs for coding standards were defined, which I'm sure helped a lot of teams to leave coding standard discussions behind.
By now we all know that by installing a vendor package you can import bugs and security issues into your project. Another common concern is the stability of the package: is it going to be maintained for a long time?
Another reason for a package to become stable is when it is used by many people.