There is inevitably going to be a license involved. Server-free backup is a complex thing from a coding perspective, so that is going to come along with cost when you purchase the license. The dedicated server or disk array that you need to do this isn't free either. So, the big downsides are complexity and cost, but there is a risk involved as well.
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For example, if you are doing server-free backup you may have logs in three different places. You might have logs on the client that's being backed up, the backup server that's transferring the data and the main backup server that's controlling the whole process. So you have logs in three places that you need to check, and when things work great they're great. But, when things don't work it can be rather complex to figure out why.
Check out the entire Server-free Backup FAQ.