I'm not a KaOS user, but i have been reading about it, and i would like to see a debate about KaOS, so I'm going to give my opinion here, to see if a learn more things with you guys.
The fact that KaOS uses only one toolkit probably have these advantages:
a) Less consumption of RAM;
b) Less packages installed, which, for the final use, means less vulnerabilities to be explored;
c) Good visual integration.
About "a)" i haven't done tests personally, but i heard from a KDE developer that nowadays you don't get so much extra RAM consumption by mixing the toolkits.
About "b)" i have to say, it's a true argument, but also a "technical preciosity", few packages more or less doesn't kill you on a desktop daily use as long as i know. And to be fair other distros can have more people reviewing packages, which increases reliability on my opinion, KaOS is small, which probably means less people reading the code.
The focus on one DE for my is a double-edged sword. It's really cool to have a community focused on only one D.E but at the same time, if the upstream is passing through a really bad time, and you are a simple user, then you have a problem. On the other hand, Linux is freedom, so there are many ways to deal with hard times on Linux.
The radicalism in using only one architecture for me is something that makes the life of the developer easier. Period. I can't see this as a good thing for the user who needs a 32-bit driver for a printer for example. But i have to say, Flatpaks are helping at least with the applications.
Limitation of packages can be a good thing actually, avoids unnecessary works by the core team, and lost of time by the user testing craps. At the same time, this is something that sooner or later the software centers are going to mitigate. It's a matter of choice here, as much as focusing on package stuff.
I really like the fact that KaOS is independent also. Companies helps Linux world, but they also, sometimes, need something to be used or tested by the community, that's is when the user get's a not very pleasant defaults.