Yep, last year I compared some packages of interest (fontconfig, freetype, cairo and few others I can't remember now) and the only significant differences between Debian and Ubuntu packages were
1. Ubuntu packages were slightly more up-to-date (I compared packages in Debian 8 to Ubuntu 15.10 or 16.04, not sure which one). This is important for freetype because it keeps improving in every release.
2. Fontconfig is heavily configured in Ubuntu package. Not patched, just runtime configuration.
So there were no special patches on the Ubuntu side compared to Debian or the upstream sources. I must note that both Ubuntu and Debian's freetype package (which are almost identical BTW) enable advanced hinting options (which must be configured in compile time). Some other popular distributions don't enable those options because of lawsuit fears and this results in a much crappier font rendering that you can't fix with runtime configuration.
As I said this comparison may be slightly out of date now and I plan to repeat it after stretch is released. I didn't keep tabs on their state on recent Ubuntu releases but on the Debian side fontconfig and freetype was barely maintained in stretch cycle, so I guess Debian will still have slightly poorer font rendering compared to Ubuntu. You can still get a similar rendering by copying over /etc/fontconfig of a comparable Ubuntu release, though.
Ubuntu's solution for beautiful fonts uses non-free software I believe, and the results IMO are as good as or better than Windows or Mac.
When I switched from Ubuntu to mainline Debian, I started having to install Infinality to get beautiful fonts as good as or better than Ubuntu's.
It's a general pain point with desktop Linux, but an area where Ubuntu leads.