There's a lot of answer to that question...
By way of background, I learned on steam gauges, before the era of flip-flop radios and headsets. The avionics was simple to learn and operate, not much to it, but you spent your energies maintaining situational awareness. These days, the opposite.
One big area to consider is what techniques do you / can you / have you used on each individual instrument. I've seen the following in various glass cockpit offerings that indicated that the vendor had no clue on how pilots would use their instruments:
* Roll pointer on the attitude indicator disappeared for small bank angles. This made it harder to fly level, and hard to make corrections with 1° or 2° of bank angle;
* Lubber line on the HSI not on the top layer, so you couldn't use it, and very hard to see because it was skinny and dotted. They still haven't fixed it;
* Vertical speed indicators with only 100 ft/min resolution. Sometimes for small altitude correction, you want to climb or descent at half that or, if you're good, at 1/4 that rate. One vendor had the vertical speed disappear completely if it was less than 100 ft/min;
* Heading indicators as a linear strip across the top. That means that you get to do a whole lot of math to figure out turns;
* VOR indicators without compass roses around them. Those are mercifully rare these days as they made VOR navigation much easier;
* Lots of vendors have single cue flight directors (V bars). Those work great when you have guidance in two axes, but with single axis guidance, you get misleading pitch cues. And when you are flying attitude with no guidance, V bars are much harder to fly precisely than more explicit reference marks.
And when it comes to human factors... if you've had any formal human factors (HF) training, you can go into just about any cockpit out there and find HF errors in just about any avionics suite. Misuse of color is one of the worst offenders....
Then there's the avionics Tower of Babel. In the old days, checking out on the avionics was maybe two minutes, and a checkout in a plane focused on the airplane. These days, it's all about the cockpit light show. The two bad aspects of this are that once you learn one vendor's incantations, and that can be quite a challenge, it can be another challenge to learn another vendor's incantations. I haven't actually done that myself, but even going from what I've got to the certified equivalent when I've ridden along (with no avionics training) was daunting.
It's also been my feeling that the techniques of flying steam gauges (simple systems, pilot maintains the SA, partial panel with no attitude indicator) vs the techniques of flying glass (very complex systems, high peak workloads, lots of distractions) are so different as to justify different IFR ratings, although nobody talks about that.
Be real careful about only flying "light IFR." If you're flying from home base, you might be able to get away with that philosophy a lot of the time, but "getting away with it" doesn't qualify as good risk management.
$o there'$ really only one good $olution to your problem. $pend the time and the money to get really good at whatever it i$ you fly $o that you don't end up embara$$ing your$elf -- or wor$e.
