I can often get the gist of things in written French whereas spoken French might as well be Swahili
Very much so.
who learns spoken Egyptian (or whatever) Arabic won't have much
Never assume that Arabs are especially literate in Arabic.
About the different varieties some are highly idiomatic too. Others have a very restricted vocabulary.
Victor Mair (eminent American sinologist) said that spoken Mandarin is the easiest language he's ever learned and written Chinese the hardest..
He's probably right. Acquiring a written language that (like Hebrew and Arabic) that has sense units rather than words is bloody hard. A language like Mandarin Chinese (where someone from the northeast and someone from the southwest can't understand each others' speech but can both read the same newspaper) is wholly another.
With tongue twisters there's a phenomenon (there's a good Polish example) where speakers of a language make a tongue twister that's hard for them however speakers of some other languages don't have a problem.
I'm thinking of "butelka oetykietowana, butelka nie oetykietowana". Very easy for you and me but a tongue twister for Poles.