It will depend on your acceptance level.
Manjaro repo contains almost the same packages as Archlinux default repo.
Manjar repo differs from Arch in the following places
- Arch specific packages is not available (kernels, nvidia drivers, arch specific scripts)
- Overlay packages
- Packages existing on Arch but rebuilt for Manjaro or has a different meaning (e.g. pacman-mirrors)
- Packages developed specifically for Manjaro (some may be backported to Arch using AUR e.g. pamac)
- Useful AUR packages adopted by a team member and maintained by the team member (e.g. polybar). Such packages may be dropped without notice.
This structure leaves the possibility of human errors in packaging, coding etc and therefore the initial point is called unstable.
This indicates that errors will occur from time to time - they are rare but it happens.
That said - when I discovered the structure - I switched to unstable branch - to be as close to upstream as possible - and I have never had any showstoppers - bug yes - they are usually fixed very quickly - most of them has actually happened during my own maintenance of various applications and packages.
So while the name could signal trouble - if you accept the possible challenges - you will find the unstable branch as stable as you are.