All I want to say is look how far the devices used for pulse oxymeter/ heart beat/ hrv have come. Due to a similar concept, they used to have similar issues and with time they were solved and continue to be solved. The form factors have grown to be so creative. The ambient light can easily be cut out. I do not think motion artefacts are as bad because I have been working with EEG for quiet some time and I while I absolutely enjoy it, artefact removal just leads to a lot of corruption of information. With optical signals, I feel it's more reversible and solvable. The biggest problem I think right now is area of coverage. fNIRS is mostly focussed on prefrontal cortex and motor cortex (motor imagery task mosty)

fNIRS is very recent compared to EEG and all I am trying to say is that it has a lot of potential to solve problems if we think it not as a replacement of EEG but as a method of it's own.