Cetana bugged out again in Stellaris. That makes literally 2/3rds of the times I've encountered her a disaster that was unwinnable due to bugs.
After she establishes outposts in your territory, a bunch of dig sites are supposed to show up and you're supposed to get events telling you to destroy convoys and outposts to get intel on her and delay her progress to the instant game over. You get damage bonuses against her for completing some of these, which is almost necessary because her ships have insane stats and insane regen. They're supposed to be almost unbeatable without those bonuses.
So... I didn't get the bonuses, and found that yes indeed 1 million fleet power was not enough to beat her 8 million. The AI didn't help at all. They usually don't help much anyway, but the last expansion made the AI much worse since the cosmic storms lead to many of them collapsing from stability problems. I've never seen so many rebellions.
I don't think the devs have fixed any bugs related to her since she was introduced. It's a gamble if you plan to play to the crises like I do, since it's a coin flip if you'll just hit the bugs and be screwed. I made a backup save early into the crisis for just this occasion, but it was after those events were supposed to trigger so it was already too late. I'm going to have to stop playing on ironman, I guess. Corrupted the save trying to turn it off so I could bypass her with console commands.