The challenge of packaging Sage was primarily around packaging its dozens of dependencies (some of which I had to talk to the authors to fix their licensing terms) and making sure that an up-to-date version of those dependencies was available in Debian. It took about a month of my time to package Sage well for Debian (at the time I maintained over 100 Debian packages for MIT, so I was quite efficient at this).
What killed my Debian packaging effort was that Sage was very large and my package was submitted to the NEW queue (where Debian does copyright review) when all the reviewers were busy with managing a release freeze. So it took more than 6 months for Debian's FTP masters to fully review it, and by the time they did, I had moved from being a grad student to the CTO of Ksplice. It probably would have been just week or two's work for me to update Sage across those 6 months, but running a startup is a lot of work, and I never found the time to do that work :(.
Overall, my opinion is that Sage is well-engineered and not difficult to package given its scale (it has a fantastic test suite, so it was very easy to check if the package worked, and it was easy to get them to merge changes to improve the tooling). The problem is that it's a large project, with a large number of diverse dependencies, new versions of which aren't always backwards-compatible upgrades. If you talk to the folks who package other large projects for Debian, I'd be surprised if you find any that don't involve significant maintenance work.