I've played two games with Secret Societies now, and...I love it! My first was with Voidsingers Kongo. I found that society to be really flavourful and fun, and Kongo was a nice Kongo. Then I went with Hermetic Order Mayans, and it too was a lot of fun. I get that Voidsingers are probably more powerful, but the Hermetic Order was still fun, and I had an unusually fast (for me) Science Victory. I'm okay with societies not being perfectly balanced, so long as all of them are useful in certain situations and synergize with different civs. They add some much needed flavour and spice to the game.