If a headline is difficult to read because it is written in all-caps and the words lack form but each individual letter is clear, is it illegible, unreadable, or something else?
As Wikipedia's article on ALL CAPS indicates, it is legibility which is affected. While it doesn't state that the text becomes completely illegible, it notes that the practice leads to low legibility.
Bad handwriting is considered to be cacographic.
Its both "less legible and less readable". The words illegible and unreadable are too strong. Besides the legibility of the headline depends on the context, like a newspaper daily having its name on the top most headline of the newspaper, a form of art(calligraphy), etc. They can also mean shouting, if used in emails and such media.
Ambiguous (open to more than one interpretation--now here vs. nowhere):
Incomprehensible (no meaning at all can be extracted):
Incoherent (words with apparent meaning, but used illogically or inconsistently):