I'm glad to see this.
I really don't buy the downsides against this.
"If you have multiple copies of dependencies, all the apps that depend on it won't get security fixes when a single dependency is updated."
This seems to be a nice goal, but doesn't seem to work in the real world. Dependencies in distros seem to stagnate a bit as apps progress. Dependencies/Apps don't get updates because of conflicts so NO ONE gets the new security update because versions are locked to avoid conflicts.
I guess I'm just really tired of seeing a new version of software I use (or new software that I WANT to use) on the developers web site with lots of additions I really want. Then realizing the version I have on my distro of choice is ancient, and there are no binaries for my distro. So, I grab the code and build it. Only to find out it requires a library version that is also not in my distro. Sigh, so I grab and build that. Then realize that library won't build with the gcc version on my distro. So, I go grab that, and so on until I finally have a nice blob sitting out on /opt for the app. I am a developer for a living, and I get enough of building software at work. When I want to use an app I just want to hit install and be done.
And finally, if a library in a distro is not frequently updated, snap packages would actually be more likely to be newer, more secure, versions with bug fixes, because the developer himself can keep the package updated as he develops the app. I would much rather have 3 out of 5 snap packages with the newest code/dependencies than have 5 standard debs all on old versions because their dependencies don't play nicely with all of the apps, or the volunteer packager didn't feel like taking the time to update the library.
Now, I won't call this a silver bullet to replace all things binary distribution. But, it sure looks to fix a really big issue that is widespread on Linux. Allowing the developer himself to "easily" package his latest and greatest version for a smattering of distros seems like a good idea to me.