The Carbon Doser has a very good reputation as a reliable regulator. Yes, it is a single stage regulator. I’ve read that it uses an electronic Clippard Mouse valve that opens and closes rapidly and consistently once set. Because of the nature of how this electronic valve works, Aquarium Plants claims that it protects against end of life tank dump even though it’s a single stage regulator.
We snobby DIYers

with our Matheson, Airgas, Concoa or Victor dual stage regulators source lab grade stainless steel Swagelok metering valves and German Burkert solenoids. Often we can do this for a good bit less than the Carbon Doser. Once you go through the DIY process and understand how each part works, whether it’s fair or not, you realize that the Carbon Doser is essentially an electronic valve in a pretty plastic box married to a cheap single stage regulator. Like I said, the Carbondoser has an excellent reputation for reliability and ease of use, albeit a bit steep in price for what it actually is.
I built my Matheson dual stage for less than $250.