s2member writes loads of transients into the wp_options table. I doubt if it cleans up properly or if all are needed.
While they are not autoloaded - a huge wp_options table is not good for performance reasons. Within 5 days (low activity period right now for my website approaching winter in Europe) - I have over 1200 lines of transients in my wp_options table (the full wp_options without s2member transients is only 500 lines).
I think it would be great if there could be a review if they are really needed. If yes then it would be much nicer if s2member could just write them into another table - and not wp_options for performance reasons. I cleaned up the transients recently - but I had over 50MB of those s2member transients leftover before. Much easier to clean if they are in another table and better performance.
Since wordpress 4.9 - wordpress should clean up transients itself - maybe there is some problem with s2member transients and wordpress cannot drop them?