As you say, not much seems to make deliberate use of them as a constant feature such that you could expect familiarity to be rewarded. So, with the Functions on keyboard they are usually a lucky dip.

Soft keys specifically have an intuitively understood relationship - usually through physical placement - with the screen to link them to their purpose. The "softness" is about the keys purpose, as it can change depending on context of the screen.

A key without a intuitive link to the patch of screen, is just another F-key...