NULL is more of a declaration that 'there is no value', than mathematical value on its own. if you compare NULL with something you don't get false, you get NULL (which is often considered same as false).

some implementations carry the same "uncomparable" logic when checking for unique constraints, while others drop the ball here.

programming language wise, it's kinda like null pointer or float NaN, both of which are also occasionally complained about, eg. indexing maps by float values can do bogus things in case of NaNs.

edit: clarified some things