A) Snapshots come out far more regularly then once a month. Going from the mailing list we've actually had a snapshot released every day since the 4th. I'd say the average is one every three days but that's just from the top of my head.
B) they do not cover every package you have installed: They don't even know what packages you have installed, snapshots are cut on the repo side. It's true that packages get rebuilt a lot but that's because either the package updated or a dependency of the package updated that caused the package to be rebuilt.
There are quite a few things I've grown to dislike about Tumbleweed after using it for the past 7-8 years, but the upgrade experience is not one of them.
I'd like to know if the Gnome 4 in Leap was usable. If I do a search on software.opensuse.org on gnome-desktop it only turns out packages from tumbleweed and experimental packages from SLE-15-SP2 which looks quite ancient.
That's called a trade-off, a very fundamental concept in pretty much everything in life.
And no, snapshots don't come out every month. They come out ~5 times every week. The most number of packages I've had to update was maybe around ~3500 and that's after 6 month of not upgrading my system.
I agree for "true" rolling distro enthusiasts, Arch is still the top choice, but Tumbleweed is great for those seeking to use a rolling distro without sinking too much time into configuration.
Between those snapshots you might have bleeding edge updates for all the packages, but even then I do encounter package conflicts way too often. Well, on the upside, at least they are detected.
So yes, it is stable, but it comes at a price.
Apart from that, the community support felt mediocre, at least a few years ago. The most visited platform was a bulletin board forum with very little interaction. When I had trouble installing KDE, it took a few days until someone suggested the correct diagnostic tools. This is bad for being the testbed of a commercial distribution. In the end, I just installed Arch, which packaged KDE better than Tumbleweed did.
But on the other hand, maybe only if you use a distro long enough, you get to see the downsides, and each one has them.