Once an Ubuntu release has been cut the debs within it can (for the most part) only receive bugfixes, not new features. Which means that as an LTS goes through its lifespan, the software within it slowly gets further and further out of date. Snaps, on the other hand, are as up-to-date as their publishers make them.
>> broken dependencies.
Not that it's Synaptic's fault, but some packages will have conflicting dependencies, especially if you're using PPA's to get recent software versions. It's not uncommon to get "foo (latest) requires bar >=3.1 but you have bar 2.8 installed. Baz (installed) requires bar 2.8". Snaps avoid conflicts by having each app contain it's dependencies, which could otherwise conflict.
It also negates the entire point of having a package management system, as you can’t upgrade a security issue in libfoo and get it fixed once, but have to wait for all the users of libfoo, and the users of users of libfoo, and so on, to provide a new version of the Snap package with a new fixed libfoo contained inside it.
Yes, you don’t get that library update everywhere all at once, but this gives each vendor a chance to make sure that update actually works with their app.
I always felt the the offical Ubuntu package manager was a dumbed down version that(though its been a while sine I have used Ubuntu). Synaptic definitely felt better for installing libraries instead of applications.