To underline this, I think the sources of Upstart are really nice. It's very readable, well documented, has many tests. In many ways it's exemplary.
We didn't have any problems with the quality of code or anything. Heck, we have so much more bad code in our stack, Upstart totally stands out in quality. So, the implementation is fine, however the overall design was not, in our eyes, and that's very hard to fix without resulting in well, basically a pretty complete rewrite, and that's what we then did, after much consideration.