I'll respectfully submit a different opinion on this. I fly pretty much all of the EFIS's out there at times - and several of them very frequently. For people who are truly going to use their airplanes IFR, I am quite convinced that the greatest risk they face is remembering to push all of the right buttons in the right order to fly an approach or other procedure. On some older systems, just remembering which buttons to push to bring up which displays can be a little challenging. It's not a hit against any designs - it is just that when you need to do something that is complicated and has lots of options, you necessarily end up with complications in order to select those options.
Right now, with any of the major systems, you have one choice to learn them - go up and burn Avgas. And oh yeah, you better have a safety pilot on board (one who is not heads down trying to teach, or learn along with you). You can (and should) sit on the ground with the manual in your lap and external power on the airplane, going through the steps - but unless it is moving, with navigation data changing waypoints and the like, it is not going to teach you everything that you have to remember flying single pilot in the soup. Some of the systems do have demo modes that allow you to watch it fly, and I know that Dynon has regular classes in operating their systems (but I have not yet had the chance to take one, so I don't know if they have a truly interactive simulator).
I, for one, believe in the value of simulator training when it comes to operating complex systems. It works - and if you screw up, only electrons scream. I really think that it would be a great safety enhancement for any EFIS producer to build a desktop simulator for their system. It will take time and money, that's for sure. Some smart code-slingers could probably graft such a tool onto an existing product like Microsoft Flight Sim or one of the more serious programs for flight simulation. The truth is, very few experimental aviators actually USE the IFR capabilities of their aircraft - so maybe it isn't actually worth it. But for those who do, training and proficiency would be greatly enhanced.
Operators of complex aircraft long ago figured out that learning how to operate the systems was much safer and cheaper to do on the ground. Our cockpits are now capable of much more than those early "complex" aircraft. The market may be small, but it would be worth it to those who need to be sharp on how their equipment actually works. Head's down on a bright sunny day sliding down an ILS, trying to figure out what the "Suspend" button does, is not a good place to be - for anyone else in the sky.
Paul