it's not the architecture that's problematic. you just use a different compiler.
the operating system and graphics driver are problematic. changing every call takes time.
I am under the impression that Unity abstracts much, much of the platform difference away. But it's still an additional platform to support and implement, there is a bunch of handiwork on making sure stuff works, submit for certification, change the failures if it's rejected, submit again until it's approven, and then deployment.
Xbox One is the lowest common denominator in terms of performance, but also it's Microsoft's, so it would be more similar to Windows. If it gets through, performance-wise PS4 shouldn't pose much of a challenge.
There is still that uncertain aspect of how mods will be handled in Xbox One. It may be that Microsoft is planning something in this regard modeled after Steam Workshop. PS4 would need a similar development to be viable.
It's a safer bet (and already a large enough project) to put up the resources on a single console and see if it sticks. If the Xbox version tanks, they won't have spent resources in a dud for the PS4. if it sticks, they will have more resources to build up a team to manage the PS4 port