I agree that the Zangyack being underdeveloped and non-threatening at times is a legitimate criticism, but comparing them to the Vyram, one of the best (if not the best) Sentai villain groups ever, I think is unfair. I personally don't care much for the Zangyack, but after the last two episodes, I'm hoping we'll see the potential we saw in the early episodes, at least from the emperor. There's also Basco, who despite his lack of screentime, has been the best villain on Gokaiger so far.
Other than the villains, however, I think Gokaiger has hit the mark on everything else it's set out to accomplish. Most of the tribute episodes are well-written, the storyline is descent, the cast is solid. It's taken the writers a while to get around to developing all of the characters, but they're getting there.