My few peeves: one, you're practically THRUST into the story with little explanation. All the backstory is done in FIVE minutes before the title. "Rift opens - monsters attack - we build giant robots."
Two - I would have liked other mech than the titular two to have more screen time. The moment they get onscreen they get wrecked. I kinda liked Cherno Alpha
I won't complain about the characters: some would say they're one-dimensional and shallow, and yes they are. The key point here, IMHO, is suspending the disbelief. They're not to be seen as "movie characters".
Plus, I fell in love with Mako Mori as soon as I saw her. K once told me that moe cannot be explained. You have to feel it, and thanks to Rinko Kikuchi now I know. <3
The mech-vs-monster battles are just perfect, and the plot that ties them together, though simplistic, follows the footsteps of anime perfectly, down to the final scene.