Let me start by saying that FS can in fact be quite close to the real thing with a bit of work.
I had somewhere around 2000 hours of Sim time before I started to get my PPL. Once I had picked a flight school and knew what airplane I would be flying I got myself a copy of te PA28-161 POH and a FS9 version of the Warrior made by Rien Cornelisson ( hope I spelled his name correctly ) and tweaked the cfg and air files until the numbers in the POH were closely matched in the sim.
On my very first lesson the CFI never touched the controls...though he had the hands close by the yoke at all times.
Once I had a few real hours in the Warrior I tuned the simulated one to match the actual airplane I was flying. In the end I could replicate 2124F' behavior near as makes no difference in the Sim and greatly improve my progress to the ticket.
I repeated that process when I started flying a PA28-181 and then used that airplane to get my IFR ticket and once again to practice for my Saratoga.
The reputation of the F-104 in the Luftwaffe as a crew killer at least in part has to be blamed on the brass tasking the aircraft to play a role it simply was not build to play. It was never ever intended as a ground attack aircraft....and it was absolutely ill suited for that role. And aside from a few mechanical issues that was where most of them came to grief.
It was of course a challenging aircraft to fly with a relatively narrow sweet spot where it was happy. Fly outside of that envelope and you are indeed balancing on a knife edge. The sim versions seem for the most part replicate that quite well.
When our team released the quite complex Lockheed Constellation series we expected a few e-mails that would deserve to be answered by a short and sweet ****. We did get a few of them but since most people had followed the build at CC were avid propliner fans actually not as many as I thought and of course we did not simply say **** either.
Falcon makes a good point regarding the realism sliders. Start easy and make it progressively more "real" as you get comfortable. In the case of the Connie's that would however still leave you with some of the build in realism/failures that do simply require knowledge and proper handling by the book.
I have been lucky enough to fly a number of different types in my time and hope there will be many more. In all of those cases except those where I was only riding right seat I sat down and read the POH and talked with a pilot who had time in type to get a few pointers on the do's and don'ts for the aircraft in question.
If you approach the FS aircraft in the same way and then establish your own performance sheet by flying a few stall series up high, go through a few configuration changes and note airspeed and power settings that work in general you will get much more enjoyment out of the new birds.
Ground handling of a tail dragged is difficult.. Period. If anything in some ways FS makes it too easy.
Dont believe me...invest $50-100 and find an airport near you where you can get an hour of Tailwheel instruction. I can tell you that a lowly J3 cub will humble you just trying to taxi to the runway.
A P-51 at full throttle has way more torque than the rudder can compensate for. The most recent accident of a P-51 at KCMA is a unfortunately deadly reminder how quickly a Mustang can get ahead of you.
http://www.aopa.org/asf/ntsb/narrative.cfm?ackey=1&evid=20070720X00970
My AP owned and flew a D model with his brothers for some time and all three had learned to fly with conventional gear aircraft....and still one of them managed to bang the left wing tip onto the ground in a similar incident...but with much better outcome.
For me personally the most challenging FS aircraft is the magnificent Russian Tri-jet 152. But I have to admit that is largely my own fault because I still have not fully read that manual
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and the systems modeled are mindblowingly complex.
Cheers
Stefan