Gracefully handle edge cases. I've seen pacman continuing as normal and pretending that everything is fine, burying the error in the middle of several screens of logs, when free disk space temporarily went down to zero during system upgrade. That just doesn't happen with apt, where you're usually `dpkg --configure -a` away from recovering from most disasters.
There's also a matter of packaging practices, which isn't entirely a pacman vs. apt thing but rather Arch vs. Debian (although package manager design does influence and is influenced by packaging practices). In Arch, the package manager will happily let you install or keep an out-of-epoch package installed during an upgrade that will just fail to function. apt usually won't let you proceed with an upgrade that would lead to such outcome in the first place. It's a thing that's ridiculously easy to stumble upon as soon as you use AUR, but since user's discovery of the issue is delayed, most people probably don't attribute it to package management at all - they just see an application getting broken one day for some unknown reason, while apt screams at them and appears broken right away when they try to use apt.
To be frank, I don't know for sure that relations between packages that Debian uses couldn't all be expressed with pacman, maybe it's possible. What I know though is that I've never seen a Debian-like system that used pacman, and I know that makepkg-based tooling is very far away from debhelper so even if it's theoretically possible with pacman, you'd have a long way to get there with your tooling anyway.
There's also a matter of packaging practices, which isn't entirely a pacman vs. apt thing but rather Arch vs. Debian (although package manager design does influence and is influenced by packaging practices). In Arch, the package manager will happily let you install or keep an out-of-epoch package installed during an upgrade that will just fail to function. apt usually won't let you proceed with an upgrade that would lead to such outcome in the first place. It's a thing that's ridiculously easy to stumble upon as soon as you use AUR, but since user's discovery of the issue is delayed, most people probably don't attribute it to package management at all - they just see an application getting broken one day for some unknown reason, while apt screams at them and appears broken right away when they try to use apt.
To be frank, I don't know for sure that relations between packages that Debian uses couldn't all be expressed with pacman, maybe it's possible. What I know though is that I've never seen a Debian-like system that used pacman, and I know that makepkg-based tooling is very far away from debhelper so even if it's theoretically possible with pacman, you'd have a long way to get there with your tooling anyway.