There is the possible impact of using the tool Pull Jira user data + using a custom SSO external ID.

The connector will replace or overwrite the Jira Align user’s SSO custom External ID with the Jira External ID used during Jira Align and SSO authentication. 

Sometimes, customers accidentally click the button. The customer has requested the addition of functionality that allows them to enable or disable the user data pull button. This feature will be located in Jira Align under Administration > Jira Settings > Pull Jira User Data > User Data Pull Button.

Related Suggestion:
JIRAALIGN-6620: Help Jira Align - Jira integration setup document - > Add a warning of impact note Pull Jira user data area
JIRAALIGN-6774: Connector - Pull Jira User data should have a confirmation dialog box after hitting User data pull button