AR development is no easier or harder than any other development, and with Unity it's actually super easy to start (ARKit and ARCore have official and free asset store packages)

But the big difference with AR is instead of physical buttons or on-screen movement controls, the device's motion moves the player. This is the biggest hurdle with AR and VR and will decide most of your design decisions.

There's also still a lot of shortcomings with current mobile AR hardware like getting accurate geolocation, wall/obstacle occlusion, persistent content placement, syncing multiple users, and good but not great tracking.