It always felt to me like when people use words like "intuitiveness" or "usability", they're all pointing at slightly different things. Matt Fuller's partitioning of user-friendliness into "easy to use", "easy to learn", and "easy to use without learning"[0] feels like it might be easier to set objective criteria for. For your example, you might say functional programming tends to be easy to use, but hard to learn and hard to use without learning.

(I'm not current on PL/HCI literature, so it wouldn't surprise me if they already have these concepts, just with different labels.)

[0] https://www.over-yonder.net/~fullermd/rants/userfriendly/2