I believe it really is reliable. My EFL pre-intermediate students in the MIddle East just cannot read anything above 10-11. You just have to bear the following in mind.
1. You have to use the same analysing instrument. I use the one available in "Resources" at usingenglish.com
2. Do not rely on other people's Fog Index, because you never know what method they used to calculate their index.
In my opinion,it is ture
what is exactly he Gunning Fog Index
Thankfully to usingenglish.com i have learnt what is the Gunning fog test !