A close companion to What in your opinion is the ugliest/most unappealing script?
Cultural familiarity is going to defuse anyone’s opinion; so you won’t get many responses nominating Latin, or anything originating on the same continent as Latin.
Is it Lontara alphabet, optimised to be written on palm leaves? Is it Vai syllabary, which aesthetically occupies a middle ground between a syllabary (which it is) and hieroglyphics? Is it Ogham, which reduces letters to tally strokes?
I’m going to go with Duployan. Duployan is a French shorthand system, and Duployan was used to write down Chinook Jargon. That’s why it has been added to Unicode: Duployan (Unicode block).
As a shorthand, Duployan was designed for easy and fluent joining together of characters. What it was not designed for was straightforward rendering in either print or a computer screen. Read the rules for rendering the script in the 2009 proposal to include Duployan in Unicode, or the Unicode Technical Note Duployan Shorthand, and wince.
Like this kibbitzing site says: Crazy On Tap – Unicode 7
That Duployan by the way is an obscure form of shorthand that was proposed for a small number of weird languages, but which actually never caught on. All those languages use different orthographies now. Duployan is impossible to render using standard font engines because the shapes can combine in infinite combinations, and do stuff like the end of one segment connects to the start of the next, and they can rotate. It would be easier to implement rendering of fancy calligraphy or handwriting that looks totally real.
“Duployan orienting vowels are written by rotating the vowel to match the incoming angle of the preceding character, then mirrored along the axis of that character to avoid the following character crossing.”
Good luck updating your font renderer to handle this character range.