Biggest sites are very fair indeed, like primedice, although there are people said PD is unfair, but the proof shows PD is fair.
I think the dice is not fair because it is set on the system, so it is unlikely we will win continuously. I once played dice on a few sites and as a result most of my defeat.
if you lose then change your strategy, it has nothing to do with the game itself.
being fair means you have the exact chance of winning that the site shows you on the screen and also it means you can check to see if the number you rolled was generated random or not. you can read about it more and also check it on the sites that you have played