They test run a little more tests on their backports (and have the manpower to actually look into what is failing) and they have really far out EoL once the kernel release is selected at fixed intervals. So if you managed to get the capabilities that you care about in their tests, there is a low probability that applying updates will cause some catastrophic outage or data loss over the next 10 years. Which can be improved further by fine-tuning phased update rollouts.

Unfortunately, they did in recent history manage to still break iptables, zfs, nilfs and a bunch of machines with weird (arguably broken) firmware anyway, so my personal comparison between Debian (or simply directly the Kroah-Hartman/Levin stable upstream) and the Ubuntu backports is not exactly in favor of Ubuntu.