It all depends on what device is recording the video (in your case the iPhone X) The lighting conditions, quality of high ISO and lens apeture are all factors.
Each frame in the 60 frames per second has to be exposed. If you are in a low light scene the result may be less sharpness, although I'm sure devices such as the iPhone use algorithms from frame to frame to help achieve better quality. The only real advantage I see to 60 fps is that you can slow down the recording and achieve smooth slow motion shots.
It's not as easy to see at 1080p but if you record 4k at 30FPS / 60FPS you can see the 60 FPS footage is darker and lacks contrast. This makes sense because the image sensor has half the time to capture light at 60FPS. 60 FPS will look smoother, assuming you have favorable lighting conditions.
Higher frame rates are typically only good for one thing; slowing down the motion in post production editing. It's good for faster moving objects as well for smoother motion, but 30 fps shouldn't give you any issues for 99% of things out there that you are videoing.
As long as you're at 24 fps or higher, you're getting smooth motion in real-time video.
There are a lot of factors that go into the quality of the video that is being taken, and the poster above did a great job of explaining it.