Actually, VSS backups should be performing IO freeze and thaw. I can't say for sure whether Unitrends actually follows the recommended method for VSS backups, but if they do, they are freezing IO and then thawing it.Drawback is that VSS will not pause the writing so you can loose transactions or cause some DB inconsistencies.
Though I agree 100% with your recommendation of using native SQL Server backups and having Unitrends just back up the SQL backup drive. That is exactly how I do it (though with different backup software vendors) 100% of the time.