Is adb backup usable in case of soft-brick? Let's assume I create a backup of my phone with this command:

adb backup -apk -shared -all


When I want to install Xposed framework it warns me that the phone maybe trap in re-booting or suffer from soft-brick so they advise the user to create a nondroid backup. my question is that is adb backup method helpful in such scenarios?
A softbrick usually renders the user unable to boot into Android OS successfully and satisfactorily, and most of the  time you can boot into a custom Recovery (you would be having it if you mentioned Xposed) and then use the ADB commands to restore the backup from there. 

ADB backup method isn't useful in case of Xposed Framework since ADB doesn't make any image of /system or /data (Xposed makes changes into /system). ADB backup is concerned with user data (Apps, app data, settings, and similar things, internal/external SD card data) only. That's why Nandroid is recommended and always preferred.

Note that the command you mentioned will backup only shared storage (SD card), all apps (user+system) data and their apks as well. That's just it, but not enough to run a system.