Interesting article about the movie The Watch. They're not complimentary about the improv elements.
I can't blame them. Even though a lot of scripted comedy blows and ad libs are often far funnier than the script, that tends to happen most often for crap tv shows, particularly awards shows where the patter has been dumbed down so even the family dog can gets the joke.
But improv often doesn't work well in the long form. Theoretically anyway, when you write a script you actually structure the story; you can shape it, forge it, hone and sharpen it over time until it's at its most effective. With improve you're just winging it and there's no going back. So if the pacing is off, too bad; if someone says something that turns the plot in a different (not necessarily better) direction, so be it. The result isn't electricity, it's often just a meandering.