When choosing a spectrometer for a handheld instrument you need to pay attention to the environmental stability of the spectrometer. Temperature changes, vibrations, and shocks can potentially bring the spectrometer out of calibration, so you want a spectrometer that is robust against such effects.
One important aspect to consider is that transmissive optics are much less influenced by small angular variations than reflective optics. If you consider the difference between a mirror and a window this becomes apparent. If you are looking at something in a mirror and someone rotates the mirror a few degrees you will see the mirror image move. However, if you look through a window that is being rotated nothing happens. This is the main reason spectrometers build from transmissive optics are much more robust.
Low power consumption
The only electrical power consuming component in a spectrometer is the detector. Especially, if the detector is cooled it can consume several Watts. Therefore, handheld Raman spectrometers should be designed around non-cooled detectors.
The objective of cooling the detector for (benchtop) Raman instruments is to reduce the dark charge level which builds up over a long integration time. So, using a non-cooled detector typically means that the integration time should be limited below 1 second.
Spectrometers for handheld Raman