EndeavourOS previously received an influx of ex-Manjaro forum members due to some disagreement as to how Manjaro devs handled - among other things - repo management (how long to hold back Arch updates from Manjaro Stable, etc) as well as forum control (all fun and off topic stuff, even screenshots, were no longer allowed on the Manjaro forum).
I'm sure Endeavour is a fine Arch-repo-based distro. Manjaro is not a direct Arch-repo distro since it pulls in Arch packages and updates into its own 3 repos - Unstable, Testing, stable - and then tries, not always successfully, to manage their progression through Unstable >> Testing >> Stable repo, in order to smooth out rough Arch updates.
The grub issue in Arch was due to some changes upstream by the grub developers. It affects uefi systems only. From what I can see, Manjaro was not affected because they had previously already forked grub from Arch's.
In any case, it's not really EndeavourOS 's fault. But perhaps newer users who have the wrong expectation of various Arch-based distros with a GUI installer will have been freaked out by the grub issue, or other rough updates.
"Easy to install" (for arch) does not mean the user relinquishes the obligation to maintain the system. Updates aren't click and forget. That's the price of an "install once, use forever with the latest packages" system. But many who come from fixed release distros don't realize this.
And if Mauser is right and Endeavour doesn't ship with a GUI package installer, I think it's a good move. Let the users get used to the basics of Arch package management so that they understand the appropriate role of third party GUI package installers if/when they do install them themselves.
Manjaro is currently going through a rough KDE Plasma update because the latest version from KDE devs is buggy. Quite a lot of forum posts questioning why Manjaro would simply allow the buggy upstream version into their Stable repo when they are meant to be a distro that tries to smooth out Arch issues. While I'm neutral about this issue, I do wonder if this latest issue will again gain Endeavour and other Arch-based distros new members.
PS. I haven't had issues with my long-lived Manjaro installs, but that is because I use XFCE, which is quite stable and doesn't go through crazy-fast changes all the time, and am on MBR legacy boot.