You may find that on a screen where you can see Risks from multiple Audits (e.g. the Analysis screen), multi-selecting Risks from different Audits and attempting to make an assessment results in the following error:

A business rule has been broken when performing this task, please click below for details.

Entities: 10
HasChanged: True
IsOkToSave: False

Entity: 0 (Risk Assessment)
HasChanged: True
IsOkToSave: False
IsOk: True
Invalid Element: Inherent Likelihood
Error: Inherent Likelihood is invalid because it does not match the Configuration entered
Invalid Element: Inherent Impact
Error: Inherent Impact is invalid because it does not match the Configuration entered

This occurs where Local Matrices are in use and the Risks selected are from Audits whose parent Entities use a mixture of different matrices.  You will need to ensure that you bulk assess only Risks related to Entities that use the same Risk Matrix.