Headphone outputs can be used to drive line level loads and can provide good noise and distortion performance but are typically not as good as line level outputs.
Line level outputs typically have a source impedance of 50 to a few hundred ohms. They are intended to drive loads that have a high input impedance, usually greater than 5k ohms. They are typically optimized for very low noise and low distortion when used with high impedance loads.
Headphone outputs typically have a source impedance less than a hundred ohms and the better ones have a source impedance of a few ohms or less. They are intended to drive the lower impedances presented by headphones which typically range from 16 ohms to 300 ohms. They are optimized for being able to drive higher current and for delivering reasonable distortion and noise with low impedance loads.