From what I remember Retroelectronik had a warehouse fire and shut down. Their supergun had poor quality control, with some people reporting buttons not working and problems like that.
The ones I see recommended are the HAS, Axunworks, and minigun. I don't know if the HAS is available to order right now (the creator does a batch every few months or so). I've seen some recent reports of the Axunworks supergun having inconsistent voltage on the 12V rail, and it's difficult to get (multiple month lead time, and the creator isn't responding to messages right now). The minigun is an open source project that you'd have to build yourself.
I haven't used one like you linked to, but I've heard warnings that a lot of superguns don't properly attenuate video signals, so if you're using an upscaler you could be shortening its life or in some cases killing it immediately.