When I was running into a few VPN configuration issues with Kubuntu 17.10 (on a multi-boot Thinkpad that already had Ubuntu and Deepin Linux), I reformatted the partition and installed Manjaro KDE as a fourth distro targeting the same /home.
The AUR, as compared to PPAs in Ubuntu (or using Gdebi to install deb files) has been a good reason for me to switch to Manjaro KDE from Kubuntu. Firstly, when Ubuntu version upgrades happen (particularly in non-LTS updates), the PPAs have to be updated and reinstalled. Since these are volunteer efforts, I've found some of the more obscure Linux packages that I use not fully "supported" -- in the sense that installation isn't so straightforward.
In particular, one benefit of moving to Manjaro has been that Java 6 is still in the AUR! I have an old bookkeeping package where I need an older version (to migrate off!) and Java 6 has been practically impossible for me to install on any Debian-based distro. (I'm okay, but not a master as an Ubuntu system administrator for myself).
The Manjaro community has been responsive. As compared to the Ubuntu world, where there are a lot of forums, and then bug reporting, the Manjaro community only has a forum. There's no way to report a bug in Manjaro, per se -- and that's not a negative, it turns out to be a positive. Since Manjaro is a rolling release, the fixes really have to be in the packages themselves (e.g. I'm having some KDE issues with Libreoffice updating from KDE4 to KDE5, but at least the Manjaro team has been helpful with workarounds).
For me, I see long term benefits for being on Manjaro KDE as my main distro. I had tried Manjaro Deepin (which is beautiful) in supporting a friend, but the difference in Grub versions creates too much support work. I've put her on Deepin Linux (and update myself rarely, before I go to visit her for a tuneup).
For a bootable USB distro, I'm not sure that there's much benefit for using Manjaro KDE. For a portable USB emergency kit, I would probably use Ubuntu (although I detest Gnome 3 over Unity), or maybe Xubuntu. On all of the computers that my wife uses, I've standardized on Xubuntu LTS, because she doesn't want to fiddle with different desktops, and doesn't use much beyond the browser and standard applications. Given the VPN issues that I've had with Ubuntu 17.10/18.04, I'm not planning on updating her Xubuntu 16.04 anytime soon.