Are there specific core game mechanics that are supported by the Rift and not the Quest?

Game mechanics arent limited by the platform in this case both are functionally identical, aside from the quest being wireless so roomscale for everyond is built in. What you should be more concerned about is the performance differences. Aim for about 15% of what a desktop powered hmd setup can handle ;)

The main things you need to lookout for is that Rift runs from a desktop environment like windows while the quest runs from an android environment, since you say you are using Unity3D this isn't much of an issue since unity can handle deployment to both platforms quite easily but be aware of limitations such as data storage and inputs which might need to be changed depending on if you are deploying the rift or quest version.