What is bad about native php sessions?

I am working on sessions and I am wondering why a framework like Laravel is convinced the default PHP session implement is so bad.

To quote from the codebase:

If a session driver has been configured, we will need to start the session here so that the data is ready for an application. Note that the Laravel sessions do not make use of PHP "native" sessions in any way since they are crappy.

http://ift.tt/2kViVuU

What is so bad about the current session implementation that we have right now? I woud love some reading material or somebody thoughts about this.

Edit

Kind of strange to get a downvote about this. Would love to know why this might not be a valid question.

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