It is very difficult to give a % figure. It also depends on what you mean by "failure".
For example, when we upgraded from SP5 to SP6, the DB upgrade did not complete successfully as it needs sysadmin rights on the DB instance and our BESadmin user only was DBO of the BESMgmt database (which is enough for running the BES, but not for upgrading).
After giving the appropriate rights, we just restarted the upgrade and all went fine. Would you consider that a "failure"?