I went to some shows in China to see demos, visited four separate manufacturers, then bought one.
My evaluation included bringing specific test blanks and having the sales team or engineers configure the system and do a weld for me to take home. I labelled and compared them all, as well as videoing them during their welds, before making my choice.
The unit itself works great. The third party el-cheapo wire feeder it came with isn't properly configured and fails constantly despite our best efforts.
Versus MIG/TIG, with which I have very limited experience but also own and have had recent fabrication done with, the equipment is more expensive and less portable.
However, it's substantially faster, easier for novices to pick up, seems way cleaner and uses less consumables.
It does do seam welds.
The big issue is getting the setting right for each material type and thickness, this is really important as is your angle of incidence. If you have the wrongs settings or vary the angle too much, the weld fails.
It's a system that's therefore best for robots, but is still good hand-held.
Note you can mount a handheld unit on a robot arm and it will do the same job as an expensive robot. If you think you may do this in future, ask your vendor for extra length when you buy the hand-held, so you don't have to buy extra cables when you automate.