These multi-dish switches have been produced/marketed by many manufacturers and they all do the same job. For as many people who tell you the so-called knock-offs are junk, there is just as many who will tell you the DN SW-21 brand is also crap.
I could see the cheapers ones maybe not lasting as long in damp/cold environments etc or having poorer quality solder joints and connections inside but as far as I'm concerned if an exposed outdoor connection is weather wrapped and protected they should all be just fine. If the cheap one dies early on you, just get another one.
I know my generic (Microroyal I think) brand name SW21 lasted 2 years outside without a hickup before I stopped using that dish anyway.