Since there are two shear lines for SFIC cores, and the pattern of the control key is independent of the operating key, you have the situation where the pin is may be bound even though one of the shearlines is correct. If this occurs, marks will appear on the key, causing you to file past that key depth.
I think the easiest way to deal with SFIC is to pick to control, remove them from the lock assembly, then decode the core.
Schlage LFIC only has one shear line, so you should be able to impression those. Once you have an operating key, just cut the pattern onto a control key blank and you have a control key.
Sargent LFIC turns counterclockwise to release the core - the middle two pins are different for the control key on these cores, so it may be possible to impression to get the first two and the last two pins.
I generally only impression locks which I can't get apart. In some cases, I can pick the lock, then measure the pin height in the keyway from the picked position, then make a key from that information. If I don't have it quite right, I can look at where the marks are on the key, and go from there.