When it comes to the most frustrating aspect of the freelance life – to wit, actually getting paid for the work – this piece in The Toast nails it. (The comment section is full of additional spot-on commentary.)
The single most reliable and prompt payer I have ever personally dealt with was a comic-book company; they paid their freelancers every other Friday on the dot. They also got swallowed up in the Great Doom that befell the American comics industry in the mid-nineties, so go figure.
The worst? Universities, hands down.
(These are the honest companies and institutions we’re talking about here. Of the dishonest ones, we shall not speak, mostly because to do so would require the use of very bad words.)