I've been on keto/fat adaptted for three years and all of my times have improved, on easier, less stressful training. Sure there were a few setbacks in the first months, but the benefits were obvious once I learned from my mistakes. You are going "against the grain" (pun intended) of everything you were taught was "healthy", so of course it is not going to be easy for most.
"I tried keto for (x-weeks, usually under eight) and got WORSE!" This is like people who try to increase mileage and after less than one/two months declare high mileage a failure and they have never run worse. You did it wrong and you ignored your body's requirements to adapt properly. That is OK, because most diet advice out there is absolute crap. Your best bet is to find a good mentor or people who have been through the process, not just lone-wolfing it.
And to the morans who think that ingested carbs are the "only energy source" that your body can use, aside from being absolutely wrong on the physiology, you are like the "running is bad for your knees" slugs looking for a cop-out. FACT: Carbs are NOT a necessary component of a healthy diet, fats and protein are.
I will be charitable, because it took me DECADES to come to the reality and gravity of the lies and misinformation that we have all been subjected to (runners worst of all, since all the diet advice is always carb! carb! carb!). Then you have idiot vegans jumping in with their complete lack of experience with a healthy omnivore diet and their Jesus-complex-I'll-save-the-world agenda driving even harder down the road which leads inevitably to insulin-resistance and skinnyfat carb-dependence. I know, I was a vegetarian quietly "saving the planet while eating healthy" for two decades. The two unheatlhiest decades of my life, BTW.
Finally, you can get 90% of the benefits of keto (at least medium-term) without having to "eat keto", but to simply eat in a window of 2-6 hours/day. You will quickly find out what the quality food is, and what is junk and filler (spoiler alert: the low-nutrition food is most of the base of so-called food pyramid, the quality is at the top). THAT is where I would start most people trying to understand diet and how their body works. If you don't have the discipline and awareness for intermittent fasting, you will never succeed on any diet.