I haven’t used SongShift, but it looks like it might be a bit more robust than TuneMyMusic because SongShift says it keeps the playlists on both platforms in sync, which TuneMyMusic does not do. TuneMyMusic is a one time lift-and-shift utility. If you’re like me, and just plan to recreate the playlists in Apple Music then quit Spotify, then the one-time move could be sufficient.
After testing a dozen or so playlists, I took the plunge and used TuneMyMusic to replicate all albums and playlists from Spotify into Apple Music. About 3% of the tracks failed. There’s no diagnostics, but I assume they just aren’t available in Apple’s library. Even for tracks that succeeded in being replicated, about 6 (out of several hundred) were flagged in Apple Music as “item cannot be played”. Perhaps a DRM issue?