Are paid survey platforms legitimate, or is the whole space full of scams?

There are most certainly legitimate professionally run survey sites that do pay their members on time. There are, however, also predatory operators on the space, and they are actively targeted at individuals, who want to earn online. One basic principle to apply: any platform that claims improbable income, requires a registration fee or any form of sensitive financial information before you have finished a first assignment is to be avoided and reported.