The cartridge gland should have a flush port and vent built into the gland. Will the design seal both pressure and vacuum. Many designs cannot do this. Be sure there are no glued elastomers, especially the dynamic elastomer or O-ring that has to flex and roll to compensate for face wear and shaft movement. The glue creates a hard spot that will leak.
Stationary designs are the best.
In this design the spring or springs do not rotate with the shaft. Many of these seals are used on mixers and this is a necessary feature in a mixer application. Once the proper mechanical seal has been specified for the application, the really difficult part takes over. Someone has to do the application for the individual chemicals and troubleshooting of failed seals. Because these people are rare, we see consumers throwing seals at the application hoping one of them will work.