The keto flu for me was the feeling of starvation essentially. Very weak muscles, to the point of finding it taxing keeping my arms above my head to shower. Lots of carb craving and hunger, inhuman amounts of peeing and muscle twitching that I associate with that. I doubled my dose of daily electrolytes to try and counter & I was feeling way better after 4-5 days of it.
Energy/cognition wasn't a big problem, but then I was such a zombie before starting from the fluctuating blood glucose/insulin that I think anything was an improvement. Before I would just crash and have to eat & sleep every 1-2 hours during the day, regardless of activity. Those swings triggered a lot of other unpleasant stuff like migraine, hot/cold sweats, limb weakness/numbness, OI, insomnia etc. Not fun.
Now I am getting back to how I was before the crash that gradually snowballed 18 months ago, much more like mild ME again. I have been going to work whole afternoons at my office job, or driving around doing chores/shopping without being reduced to mush half way through. I can actually do enough activity now to give myself PEM, whereas before starting the diet I would be destroyed by the blood glucose/OI problems before being able to exert enough. It was an oddly positive feeling when I pushed myself a bit again and felt that familiar PEM the next day, it made it clear just how unwell I had become!