What is the rationale of replacing cobbler, what are the scalability issues with cobbler?

Cobbler is a well-established provisioning tool in the RPM world.

http://cobbler.github.com/

MAAS used cobbler relabeled as maas-provision to provide provisioning services for Ubuntu environments.

There is a blueprint that describes scalability issues with cobbler, proposing to drop cobbler.

https://blueprints.launchpad.net/maas/+spec/remove-cobbler

What is the rationale behind replacing cobbler with something home-written, given the established code base and user base of a well established tool?

What exactly are the scalability issues observed with cobbler?

Was fixing these scalability issues considered as an alternative to dropping cobbler and reinventing the wheel?

The full spec is only available as a protected Google document.