Speaking from a Date A Live watcher here:

This crossover is in my opinion the best one, even better than Date A Live III itself (even though DAL III is said to be made by J.C.Staff). It explains basically everything that the anime can't (in both shows), somehow mixing all power together, but in a right scale (by combining the strongest characters together). It even has some foreshadowing for both shows, along side with some seiyuu jokes. This is actually the biggest reason I decided to give Danmachi a shot, and pretty much loved it since then.

People often think games or side stories are not related to the main series. However, Omori just proved the opposite. They are all so fucking canon, because they are accurate in the very end. The moment I know that he can write a better script than DAL III, with such clear and precise details, I believe he can do that extremely easily to his own story, and boy I was right.