Saurik (aka Jay Freeman) has just rolled out a new maintenance update for the Substrate Safe Mode in Cydia, which offers a notification dialog-box detailing the issue, whenever there is a crash while the device is booted into safe mode. Furthermore, the restart function should now work as expected after the crash.
Interested users may go ahead and update the Substrate Safe Mode tool to the latest build via Cydia's Changes tab after refreshing the sources list. It must be noted that the updated tool works only with the latest version of the Yalu jailbreak or Yalu102 beta 6 or beta 7, as they are the most stable iterations available at the moment.
As Saurik describes it, if you attempt a restart on the previous versions of the jailbreak such as Yalu beta 3 and earlier for iOS 10 – 10.1.1, it could lead to system freezes and device lock-up issues. Consequently, you will need to force restart your jailbroken device to get it working again and then boot it semi-untethered to restore jailbreak access on it.
Besides, if you are not a developer and don't have access to an annual certificate, then you will be forced to re-sideload the jailbreak app via your computer using Cydia Impactor.
As there are some known issues reported with certain beta versions of the yalu jailbreak, we may have to wait for a bug-fix update or the final public release of yau102 with necessary bug fixes.