See SCIM spec and optimization details below:

• The only operator currently supported is eq
• While filtering should technically work on every attribute of a user, it is a very slow process for non-optimized attributes. We strongly recommend filtering only on the following optimized attributes whenever possible:
  • email / emails.value / workemail
  • username / displayname
  • externalid
  • urn:ietf:params:scim:schemas:extension:lucid:1.0:User.productLicenses.