DexNav can be reliable, or can't be. It just depends on the luck you have and the odds are 1/512 like the friend safari in Pokemon X & Y, so it isn't chained and the shiny charm dosen't affect the odds at all. Even though you don't have a shiny charm, I would do Mesuda Method over this any day because you can control IVs, and if it has it's hidden abilty or not (if you are using the parent that has a hidden ability) also if you have a cute charmer at the front of your party I think, once again, I think, that it could increase the chances of it being the opposite gender of the cute charmer. You can also controll if it has egg moves. I would personally use the DexNav to get a perfect parent Pokemon and use a good foreign Ditto to breed with to get a shiny.