Agreed, especially about simplejson. I work on a project that uses simplejson, and it leads to ugly type checking all over the place because you never know what your JSON string got turned into. For example:

https://github.com/openstack/swift/blob/39c1362a4f5a7df75730...

and https://github.com/openstack/swift/blob/39c1362a4f5a7df75730...

and https://github.com/openstack/swift/blob/39c1362a4f5a7df75730...

and many more just like those.

The worst part is the bugs that appear or disappear depending on whether simplejson's speedups module is in use or not.