What is the load factor? It is rated for a SWL of 600# but is it tested at 5:1 or 15:1 or??? Is the limiting factor the 4 screws holding the side plates, the ratchet mechanism, the axel or the rope?
Also what is the rope - looks like polypropylene. Replace with spectra (pricier) and have a 15:1 safety factor with the same rating and could be great for backcountry SAR and meet NFPA specs...
I'm not sure it'll meet all applicable standards; OSHA 1926 is possibly applicable to this (fall arrest systems), and requires that all metal devices be made of steel, but NFPA 1983 (life safety rope and hardware) requires that "hardware" be made of cast, machined, etc., metal - does not specify steel vs. aluminum.
While the carabiners are almost certainly tested to applicable spec, the bringalong device itself also has to be tested to almost 5000ft/lb of force under the 1983 standard. The slings have to be tested and labeled in a very specific fashion, even the labels have to meet certain wear characteristics. Having been involved with small company product manufacturing, I know how daunting these sorts of procedures are, especially when you look at costs. The mom and pop have a hard time justifying it.
This tool surely is cool, and with the block can really have some great application with its strength:weight ratio. I may very well pick one up, but unless the owner gets lucky (maybe Darley wants to license his product?), I think he'll have a hard time getting it to pass the NFPA muster. Which is a drag....the size/weight/and US manufacturing look fantastic!