The different power systems also pose a problem. The chakra in Naruto and the artes and Therion powers in Tales of Berseria are very different. You need to figure out how they interact, if at all. If you make them too similar, it will seem unoriginal, but if you make them too different, it might be hard for readers to understand how the characters from one universe can keep up with the challenges in the other. It's a delicate balance to strike when writing this kind of fanfiction.
Another challenge is creating a consistent and engaging plot. You have to find a way to merge the two universes' storylines, magic systems, and power hierarchies. For example, how do you make the ninja ranks in Naruto work with the different classes in Tales of Berseria? And how do you incorporate the complex backstories of both universes into a single fanfiction without it feeling forced or overcrowded? This requires a lot of planning and creativity.
One challenge is getting the characterizations right. In Naruto, the characters have very distinct personalities shaped by their ninja upbringing and the shinobi world's values. In Tales of Berseria, the characters are from a more medieval - fantasy setting with different moral and social norms. Blending these two different types of characters in a believable way can be tough.