[Quoth]

Certainly Open Dyslexic was an art project with no actual study of dyslexia. Very many (nearly all?) supposed aids for Dyslexia have no scientific basis. Someone here recently posted links to actual studies debunking Open Dyslexic font.

The sans vs sans-serif readability is contentious and results seem related to size and type of content readers are used to. Those adults reading mostly novels "prefer" serif (because almost universal) and those mostly reading web sites "prefer" sans, because that's what web pages mostly use, for historical reasons.

I'd guess no-one wants to read traditional German/Gothic "black letter". It's hard to understand why it was even adopted as it's later than almost all clearer font faces.

My tests with Dyslexic readers who had never used ebooks suggested what they liked was being able to adjust font size, line spacing and margins on eink. As do most older folk! None were impressed by Open Dyslectic font.

All children and teens keen on reading novels tested (not Dyslexic) found a 4.7″ eink non-touch ereader with Georgia better than an ebook on a 6″ phone or an iPad. Oddly most liked smaller text!