The official Unitrends answer is a usually a resounding NO. Unitrends' stance on WAN backups is to have a device or UEB at each location, that way they can cap the outbound bandwidth, add compression, and send only deltas after seeding.
There is no way to cap transfers natively between remote sites with the Unitrends software, so every time you run a backup you will be flooding your connection for that site.
Unless you have at least 45mbps between all of your sites, with rock stable connections, this is highly not recommended, and if it were to be implemented and you have a site that can't seem to finish backups because of an unstable connection, the answer will be "We technically don't support WAN backups, you need to purchase another device for that location"