The /sbin/reboot is a regular executable in the filesystem.
If your filesystem is hosed (e.g. filesystem driver hang, SATA hardware hang or disk firmware hang), the changes for successfully executing that executable are slim to none.
On the other hand, assuming that you have a root shell open already, echo b > /proc/sysrq-trigger does not need any filesystem access.
简而言之，echo "b" > /proc/sysrq-trigger
可以确保环境在挂文件系统的时候重启成功；