Unity is super-fast to develop for, including both games and creation of responsive interactions. Hence, it's attractive for all sorts of XR, especially where one doesn't want to dive in deep to visual reproduction, but primarily to build a set of interactions in the experience.
Unreal is super-flexible; it can be or control anything. Definitely slower and more complex to develop. However, it can reproduce a completely believable jungle in your living room. No other real time graphics come close, and it can create something that is amazing, solely because of the visual art it enables.
In terms of areas of opportunity, it's a lot easier to forge new frontiers with Unreal, while Unity fits more naturally within areas that have been developed to envelop its functionality. -I think that's why it tends to hit something like Hololens first; if the platform welcomes it, it's a very easy entry for those exploring the technology.
It doesn't make a great deal of sense to consider Unity for virtual production outside of pre-viz though, as it never provides a photorealistic image, especially not in real time. No technology can do everything well, so that's not a dig. -I've formed this opinion not from my own lame efforts, but primarily on what Unity and Unreal themselves put out as best-use examples. Although it was indeed my own lame efforts that got me started on the research.
