Yes I think you mentioned the relevant points here. Ubuntu tests their preinstalled software, while there is tons more in the repos that is not as tested. Same with Mint.
And they backport only stuff they think is necessary. For example Plasma 5 is based on the EOL Qt5 and backporting things to Plasma 5 is nearly impossible as you need real Plasma devs and nobody really wants to do that.
Plasma 6 is really stable, 6.1 not so much, but the timing was not perfect. Simply because they do their release schedule as fixed as that.
It is a total pain if you simply want working software, as they may backport some stuff, but all the stuff not preinstalled, or that is very complex, will not get fixes.
This is the same with all stable distros, if the maintainers dont literally maintain all the software there is.