Watch out for glued in faces. Good designs will rotate the fluid to prevent a slurry from wearing the thin metal plates. Be careful of stationary seals in high temperature applications. The flush connection will produce uneven cooling causing face distortion. High temperature applications utilize heat treated bellows materials.
Some designs use glued elastomers.
This causes a hard spot in the elastomer that will not seat properly causing leakage. This can be a problem when you have a mixer application to seal. Some elastomers are not available in split O-rings, so watch out for availability. Many outside designs move towards the product as the faces wear, making them ineffective in slurry applications. Other problems that can occur shortly after the seal is installed.