I'm running these two myself, and I can confirm-- it's absurd.
For those unaware, Nino naturally gets the powerful Gronnblade tome that makes her deal extra damage for every buff she receives, and Eirika is uniquely capable of passively giving allies two buffs at once thanks to the effect of her unique weapon, Sieglinde. Have Eirika buff Nino's already high speed and attack stats and you have a monster of a unit capable of double attacking most of the cast with criminal amounts of damage behind each hit.
Nino has always been my girl since the beginning, and when I got Eirika...yeah, she makes a perfect partner for Nino. They just rekt everything together.
Nino's attack get stronger the more buff she's getting, and not to mention her already high attack and speed status. Also they looks super cute here, like sisters. <3
As suggested in a previous comic, it's WAY more powerful when you cannibalize Nino to someone else who can use the Gronnblade tome. In particular, if you can pass it to a cavalry or flier (which would mean Spring Camilla in this case) unit, then those "Fortify Cavalry/Flier" and "Hone Cavalry/Flier" skills become game-breakers. A potential +30 Atk and +6 to all other stats if you lay both on a Gronnblade user. Beyond that, just inherit Reposition and Swap onto your buffers to keep your nuke out of harm's way as you delete one enemy per round.
I see Nino as a speedy and easy-to-use Gronnblade user that needs few buffs to double and kill most units, while Cecilia (the only green tome cavalry unit atm) and Spring Camilla are the powerful and team-specific Gronnblade users that rely on the buffs unique to their own type to kill everything in one hit. Nino doesn't need a lot of skill inheriting (aside from Desperation and maybe Fury) and fits into most party compositions, Cecilia and Spring Camilla reward all your skill inheriting and team building with easy-to-use murder machines, and all three are extremely capable of slaughtering everyone in sight. Saying that it's "WAY more powerful" to cannibalize Nino for a Cavalry/Flier team is an overstatement.
Pile of corpsesYou're so strong, Nino!The Nino & Eirika combo is strong...FufuEheheThanks to you being by my side, Eirika!