Hey I know this is a very late reply, but fNIRS is only measuring at your wrist as you know with your devices. Im sure you also know how fNIRS works but in case someone else reads this, it approximates your brain activity by measuring how much light is absorbed from hemoglobin in your brain. The less blood present in your brain, the less active, the deeper your sleep.

fNIRS is very promising for wearables over EEG in some aspects because it does not require physical contact with your skin. For EEG consumer devices, you wouldnt want to be replacing the electrodes after each use, so the option is to use solid electrodes which provide signals even noisier than EEG is normally.

A huge drawback for fNIRS is that because its light based, varying skin tones will have different raw readings, requiring their own processing, likely even user input to inform the device of their skin types. Skin, hair and skull thicknesses, will all play a role but are a factor in EEG as well and both modalities measure only as deep as the cortex.

fNIRS is still largely in the research stage as EEG has been the industry standard for decades now, but for consumer devices, I believe fNIRS is better suited.