I've run across two bikes with chainstay mounted derailleurs. Here's my Peugeot PH-60:
Simplex derailleur with single cable and spring return
This is a different design than Nivex, but all of that chain wraparound makes it a pain in the rear to take the rear wheel on and off.
And if you think that was different, here's a SunTour S-1 derailleur bit brazed onto a chainstay for @
rhm
Lookit that DIY jig I made! It's actually pretty accurate, I used my flat table to check parallelism and met the spec'ed offset from the dropout.
SunTour realized that wheel removal would be an issue, so they included this little piece to hang your chain on when removing the rear wheel:
Bolts onto the old derailleur hanger. Yes, I modified the position this, but that's another story.
Price is high due to very low volume. If Jan could sell 10's of thousands of these it would be a quarter of less of the cost.
I think the biggest issue is needing to modify a frame to bolt it on. If you dig a bit deeper you'll find a $139 jig to accurately place the attachment bit. I couldn't find any spec's for the location to make my own, so one is stuck with having to purchase the jig.
Just noticed that the pullies appear to be 3D printed - if you look closely you'll see what looks like "sedimentary" layers, typical for the layer by layer 3D printing process.
If Jan had enough "mad money" for this passion project, it could be worth it as a marketing ploy. It's his business. And right now his initial run appears to be sold out. I have no idea how many he sold, or what his break even point is.