Hopping in straight to the last page because TL;DR. Some thoughts:
This could have been condensed into a two-parter, but that not what they wanted. The Supergirl episode was really well-done (surprised, but Supergirl is my favorite of the four, as of it's S2 premiere), but so barely applicable to the overall story.
The Arrow episode was essentially filler. Half of the team is kidnapped at the end of Flash and all they do is escape without significantly advancing the story. The wife and I skipped season four of Arrow after its premier, and even though we watched the S5 premiere and this episode, we're not likely to hop back on.
The Story seemed to be a letdown. Too much time spent getting Kara out of action, the final battle was underwhelming. Let's all gather on a rooftop instead of blasting the heroes from our spaceships. The choreography of the actual fight made little sense. Watch Vixen engage elephant mode and then do a bunch of spin kicks, just like a real elephant, only to have a bunch of Dominators run right into her kicks. And another Dominator turns away from Ollie just so Ollie can shoot him in the back.
The topper is that they then proceed to call themselves Earth's Mightiest Heroes, an epithet long used by Marvel's Avengers, as if to congratulate themselves, or to say that they've accomplished something worthy of the Marvel films. It's very self-indulgent, and ridiculous. Berlanti and his folk are better than that.