Well, the playlists are as save as the service is… (and its contracts with the rights owners; see the grey songs in Spotify Playlists). Alas, there is no legal way to easily backup the songs on your playlists. In SoundCloud’s case, you’ll at least find some songs you can download (whenever artists activated the option). It’s a manual one-by-one process, though. The alternative is to replicate the playlist on another streaming service. Or, to simply use the most reliable method and buy the files.
Due to the exclusive content on SoundCloud (e.g. DJ mixes, SoundCloud-only releases etc.), its likely that none of those approaches will get you all the music you playlisted on there. Some music simply doesn’t exist anywhere else (legally). Only because of that last fact, I’ll give you another hint. A web search would likely reveal that some software allows to rip music from SoundCloud (aka it downloads MP3s). Using it, however, isn’t legal in most places. I don’t approve of it.
All that said, I wouldn’t worry about SoundCloud simply disappearing overnight. Its very likely that the service would get bought, should it run out of money. Even though SC is not a good business at the moment, the platform still is an asset. Nobody is going to pay the $1 billion SC asked for, but it definitely has value.