Being that I am picky about my playlists, the ONLY 'efficient' (read: not time consuming) method I can even find is to use Windows to create a directory list of my music folder, then I manually remove the files I don't want in the list and save as m3u8. Clunky but at least gets it done in about ten minutes, compared to a good hour or so of "long press, Add to Playlist, tap, tap, back, tap, tap, long press....."
I've been using the app for years and love it, but the most frustrating aspect is playlist generation. If I have even a couple hundred songs, making a playlist is infuriatingly slow. I only want a couple songs by each artist in the playlist (such as a 'work safe' playlist -- some Smile Empty Soul songs are safe, some aren't.) I have to either add entire albums and remove the songs I don't want, or add each song individually, which is incredibly time consuming. Just the ability to tap each song I want to add and hit one main "add to playlist" would be awesome. The way I'm picturing it is,