The FD basically puts the stress of torque and weight on the tire. Other systems are bending, pulling, twisting, and inducing fatigue on vital parts not meant for it.
I stopped using gear systems because I was spending considerably more time fiddling with it and not enjoying the money I spent, and having to keep buying upgrades for a flawed system.
The FD I have has allowed me to use it as reliable transportation and allows me to focus on traffic, not the chains and gears churning under me.