'Buildings receive Gold Adjacency from resources'. That's all you need to know about Mississipian starts. If you pick them in Antiquity you can go ham with Urban Centres and the rest is history. It's Carthage with Cities.
For all intents and purposes, the Mississipians are a stepping stone for the stronger leaders in the game (Augustus, Tecumseh, Isabella, Himiko, Catherine, Machiavelli, Tubman, Ashoka, Pachacuti, green Xerxes) to do whatever ridiculous things they want to be doing even faster than they normally would. It's a heinous amount of gold (larger than Aksum's already *very big* gold gains), and it lasts until the very end of the game.
I think they're near the top of the Antiquity age power rankings imo. Not ahead of Maya and Greece, but probably on par with Han (and I have a very high opinion on Han).