There is some opinion that aftermarket electric fans aren't nearly as reliable as OEM electric fans. And when they fail, you get no cooling at all at low vehicle speed. When a mechanical fan clutch fails, it typically fails in the engaged position- you get tons of cooling just a lot of noise.
I haven't seen anything particular about the Flex-a-lite fans.
Also read several cases of an electric fan not cooling as well as the mechanical fan it replaced. However, I wonder if the electric fan was not used with a shroud. A shroud is key for the fan to actually draw air through the radiator.
I have no doubt that the Flex-a-lite kit would work great. However, $500 just seemed like a little much when I could also fix my problem for $60 for a new fan clutch.