Please excuse the double post. As the information pertains to a number of aircraft I hope I'm allowed to bump the topic.
Something I've been thinking about, but couldn't remember when making the list, is the radio altimeter.
The principle behind the device is relatively simple: A transmitter uses an antenna in the wing of the aircraft to send a radio wave towards the ground, where the wave bounces back up towards the receiver antenna.
The time this takes is used used to calculate the distance.
The problem with this technology is its reliance on aircraft orientation. To get an accurate reading, straight and level flight is required.
This is because when the antennas are not pointing straight down, the distance the signal wave has to travel is longer.
Modern radio altimeter measurements are taken from the fuselage, so it's less of a problem there. Along with its centered mounting, some aircraft also have the antennas on a gimbal, so they're always pointed straight down(with limitations of course, usually 40° pitch/roll IIRC).
However, WWII era german aircraft with a radio altimeter have the antennas mounted in the wing(either left or right, depending on the aircraft). This means when the aircraft rolls left with the antennas mounted in the right wing, the distance measured increases, and vice versa.
In IL2, this isn't taken into account at all. Even when flying inverted(with the antennas pointed toward the sky, I.E. having nothing for the radio waves to bounce back off of) the exact distance to the ground is indicated.
One thing I haven't tested is whether the measurement is calculated from the bottom of the fuselage, either wing or even the center of the aircraft model.
This is not exclusive to the Do-335 by any means. I've tested a number of aircraft which in real life carry the FuG 101a radio altimeter(BF110, Ju88) in IL2 and all of them exhibit this behavior.
Another thing I've come across in multiple aircraft is the lack of lighting on the artificial horizon. Even though most of the instrument is lit, the "bar" indicating aircraft attitude is not.
This is very problematic on dark nights, especially if there's no moon.
I would argue the entire instrument is rendered useless in this situation, as the actual indicator becomes invisible.
The following screenshot illustrates this problem perfectly:
With quite a bit of effort you can still make out the indicator running diagonally trough the instrument. However in anything but a static screenshot it's pretty much unreadable.
A similar bug, relevant only to the Do-335 also exists with the prop pitch gauges. I've detailed this in the main post.