Is the FlightGear sim accurate in 2018, specifically Cessna 172 aircraft?
It completely comes down to what you want to train in the sim. I have used FlightGear in my beginnings of when I started my interest into aviation. I guess you are using the default C172. What you can do in this sim is learning to navigate, use the instruments (at least the ones that are modelled to work correctly) and maybe learn checklists. But that's it actually.

If you really want to train for a real Cessna 172 aircraft, I can highly recommend getting Prepar3D and the C172 by A2ASimulations. It's one of the best flight simulation addons in the flight sim industry at this time. A full simulation of everything from oil viscosity to water contamination of fuel. Even a real C172 walk-around is implemented (with fuel inspection, tire inspection, everything) and a full damage model. You have to treat it like in real life to not damage parts. And you can also install a GTN750, GTN650 or GNS systems in her, if you own one of that beauties in real life. Also study level of course.

What you can't train in flight sims, however, is the hand flying. 

I own all those addons and can't stop recommending it.