Enterprise gets flak because it's "not very good." Upon watching the first several episodes, most of them fall squarely into this category.
Fortunately for the show itself, it did eventually "get good." Very good, in fact.
But not enough to save it from cancellation in 2005. A very dark time for sci-fi on TV in general (the whole decade).
If perhaps it had started out good and gotten better, then maybe it would be universally hailed as excellent Trek by all. Like TNG (wink)
That is the flak-causality. If you have an more questions, please feel free to PM me.