I remember a rant by a writer doing a Spiderman tie-in. If they would take 30 days to approve your plot and give you 6 months to write it instead of vice versa the books would be better. He said the secret is caffeine, enough that you just hold your hands on the keyboard and the fingers twitch.
Tie-ins don't pay royalties but they pay the writing fee promptly and often have short deadlines so you get paid soon. This often matters to people that make a living writing.
