To add to that... both the FFXIV world and the NieR world have a history of other beings from other dimensions/planets/planes of existence somehow finding their way to Hydaelyn (or NieR Earth). Midgardsormr and Omega come from two other planets. Migdardsormr managed to cut a deal with Hydaelyn to let his kids live on her; Omega managed to get itself fixed until it could figure out how to get stronger to get home. Ultima, The High Seraph came from another plane of existence. She then introduced Auracite that corrupted the people it came in contact with and turned them into monsters. And Ultima also wanted to take over the world.

The Red Dragon and the White Giant from Drakengard similarly crashed into the NieR universe and caused a lot of havoc. The Red Dragon was killed and released magic into the NieR Earth which wasn't equipped to handle magic and caused a lot of problems. A similar thing happened with the White Giant, only worse. Exposure to it corrupted people and more or less made them make a contract with it's god back in it's home dimension or turned them into salt. If people did make a contract with the gods of the White Gaint, they were brainwashed and ended up turning into monsters more or less... The gods the White Gaint makes a connection to are also... really hate humanity because it's took chaotic for them...

Actually... Ultima, the High Seraph and the White Giant have a lot in common with both how they got to the worlds they are in and what ended up happening once they got there... I did not need that mental image...

Point being, both FFXIV and NieR have enough inter-dimension stuff going on with key characters and at key plot points for "universe hopping" to not be a deal-breaker to me. If it was other Final Fantasy games or other game cross-overs, I'd be a lot more skeptical about it...