It freezes a point in time that you can then make an image of without worrying that files will be broken from being backed up while in use.

You don’t directly copy VSS to the new disk, it freezes a point in time that you can then backup with other software.

Veeam Agent itself is free, they do have lots of other stuff that is paid though.

Does linux have an equivalent of VSS on windows? I always thought it was odd that Linux needed to be offline to do a disk image whereas on windows I can just do it without rebooting or anything.

Yes of course, any software that makes a full image of your drive will do this. dd is the old school and very manual way of doing this. But there are programs like Veeam Agent or Rescuezilla that make it much easier.

Been happy with unifi stuff for a long time now.

Damn those are some serious limitations for an AP.