S off will allow you to change cid (carrier I'd) of phone, you can make it super cid which will allow you to flash a y firmware you like for any m8 varient.
You will also be able to flash the adsp hk update for hk sound to work fully on models that the normal mod won't work
It will also allow you to switch between GPE and sense so long as you have a ruu image for each
It will also allow you to totally brick your phone if you don't understand what your doing
Better stick to HTC dev unlock as Rom and kernals can be flashed no problem , if it's a sense device GPE, cyan, asop and sense roms should work fine
Only issue you might get is Verizon or other USA specific
Sent from my HTC One_M8 using XDA Premium 4 mobile app
.. HTC One M8