I don't really know what prevented me from getting a premium account. I found Soundiiz having a better level than other apps I tried, but I didn't really know what it could give me if I paid. Now I migrated to Spotify and I don't think that I need Soundiiz anymore, but it's still a great product!
I've used it with spotify, qobuz, youtube, soundcloud, tidal, deezer... Not only have I been able to quickly transfer playlists, but it can also keep all those synchronized with the (cheap) premium subscription, so useful!