The problem I have is with the installer GUI. They often don't work well when doing complex partitioning or mounting. Theoretically, you could use fdisk/parted on the live CD to do the partitioning. But the mounting section of the GUI (the part that creates the fstab) still struggles to map these new partitions the way we want it. This happens often when using btrfs subvolumes, LVM, dmcrypt or standard/custom ESP mount points (individually or in combination).

None of these are a problem when you are using a regular terminal shell to install the distros. You can just write fstab manually the way you like. This is a classic example of GUIs being convenient, but CLIs being more complete and powerful.

Theoretically, it's possible to achieve CLI installation for other distros too. Debian, for example with debootstrap. However, those procedures aren't as well documented as for Arch and Gentoo, because you're expected to use the GUI installer. CLI installation just feels natural in Arch and Gentoo.

Another issue I have is with boot loader installation. I have 2 Linux distros (for genuine uses) and a BSD installed. I use rEFInd to manage them. GUI installers replace rEFInd with their boot loader. While this can be reverted manually, it's annoying. But Grub has a CLI option to disable this (--no-nvram).

Does Arch have other tools beyond that which are unique to Arch?

Arch and Gentoo has additional small utilities like pacstrap and eselect. They're not big, but are very helpful when you need them.

Is there a difference how you configure a window manager on Arch and Debian?

I always find it easier to configure things on Arch than on Debian. There are two reasons for this. First is that Arch has an extensive wiki written with the assumption that you'll customize things (which is actually helpful even for other distros). Second is that software on distros like Debian are heavily patched for system consistency, while Arch and Gentoo provide mostly vanilla packages. This means that user documentation from the upstream software developer can be used directly on Arch and Gentoo, whereas you need to be aware of the patching in Debian.

One interesting example of the last point is the recent xz backdoor. That backdoor wouldn't have worked if Debian and Fedora didn't patch OpenSSH to talk to systemd. While Arch and Gentoo also reverted these backdoors, their OpenSSH were never patched and didn't have this vulnerability.