There always were 3 storytellers so I dunno what you mean with the "new" AI honestly. And they don't really scale in a traditional sense of difficulty. Phoebe is more "sandbox" friendly for those who want to spend more time building their colony than dealing with conflicts as she triggers bad events less frequently, but the conflicts she does eventually give isn't necessarily less challenging. Cassandra provides the most balanced and consistent scaling in difficulty, but that more consistent pressure also means she's more difficult later on. She's a good choice if you care about the main scenario of getting off the planet, but perhaps not the best choice for a "forever colony". Meanwhile Randy is the chaotic and unpredictable storyteller, his difficulty is never consistent, he can throw you some real overspiced scenarios at times, but he also gives you a lot of good opportunities and minor challenges too. Only reason he appear more difficult than Cassandra is because he can chain nuke you with harder challenges right off the bat if you're unlucky.
What really determine the difficulty is the other difficulty options rather than the storytellers. If you want it easy you pick Peaceful or Community Builder, you then get smaller raids, better yields, and more loot regardless of the storyteller. What biome you chose to land in also affects difficulty, Jungle biome require you to deal with deceases a lot more frequently for example; the classic Rimworld experience is the arid biome which is one of the easier biomes imo. Also, be selective with the pawns you accept into your colony early on, most negative traits can be addressed in some way or another, but you don't have that luxury early on - accepting a bad pawn might as well result in more work for your colony if he go crazy half the time. Beyond that it's important that you keep scaling your defenses alongside your wealth creation and prepare ahead for any possible scenario.
What really determine the difficulty is the other difficulty options rather than the storytellers. If you want it easy you pick Peaceful or Community Builder, you then get smaller raids, better yields, and more loot regardless of the storyteller. What biome you chose to land in also affects difficulty, Jungle biome require you to deal with deceases a lot more frequently for example; the classic Rimworld experience is the arid biome which is one of the easier biomes imo. Also, be selective with the pawns you accept into your colony early on, most negative traits can be addressed in some way or another, but you don't have that luxury early on - accepting a bad pawn might as well result in more work for your colony if he go crazy half the time. Beyond that it's important that you keep scaling your defenses alongside your wealth creation and prepare ahead for any possible scenario.