I know this is old, but it still comes up on top on Google, so I thought I would add my comments. I am currently in the middle of assembling my Waring harp, so I can’t comment on the sound quality.
However, the harp is not all cardboard. It has a wood triangle for the frame. The soundbox is cardboard, but sturdy with the addition of paint. If something happens to the soundbox, you can get another for $5.
Frankly, I would rather risk a $5 cardboard soundbox with kids (and then they won’t feel bad), then a $300 harpsicle (and definitely not my dusty strings). The harp is meant to be used by kids.
I’ll also note that so far the assembly is easy. I suspect that putting in the tune pins will be a challenge. And of course stringing is it’s own adventure, but you have to replace strings on any harp.