No, it isn't. The human body developed to bounce in and out of ketosis all the time. It's only hard to enter if you are always in a fed state and rarely in a fasted state. When you're fasting, it's not particularly hard to get back into ketosis in the same day, although it can take 24 to 48 hours for someone who isn't getting even light exercise. That's still way, way faster than the weeks it can take to get into ketosis on keto. (unless maybe you want to eat mostly protein and little fat, which is miserable)
I speak from extensive experience. A few weeks ago, after a 72 hour fast, I chowed on a pile of salmon sushi, measured my ketones throughout the day with ketone test strips, and was back in dark purple by the end of the night. Though normally it takes until the next morning for ketones to start showing up in my urine again, I believe it happened much faster because I was walking a lot that day. That was impossible to do back when I was just eating strict keto macros.
Anyone who doesn't believe me should try themselves by doing a few 72 hour fasts and refeeding on high protein, moderate carbs. They'd be surprised that it doesn't take an unreasonable amount of time to get back into ketosis. It's a perspective changer because it shows that occasionally eating carbs socially isn't the end of the world, because you'll be back in fat burning mode within a day or so.
urine test strips only measure acetoacetate. Urinary ketones often correlate poorly with serum levels because of variability in excretion of ketones by the kidney, influence of hydration status, and renal function. (Wikipedia - I’m being a bit lazy).
I’m not arguing against fasting, or low carb diets. I’m making the point that ketosis isn’t as easy to “bounce” into as people think (though it doesn’t mean there are not benefits from fasting or low carb diets).
And yes, you do bounce in and out of ketosis when you fast long enough, and it really doesn't take that long. Once you've expended your glycogen and you've digested all your food, your body doesn't have a choice but to dig in to fat stores. Depending on how much glycogen you have stored, what you've eaten, and how long it's been since you've eaten, getting into ketosis becomes ridiculously easy because the energy your body uses has to come from somewhere. You don't need to measure ketones to have confidence in this. It's well studied. If someone thinks that ketosis is hard to enter, I doubt they've gone more than a day without food. Once you get used to fasting for longer than 48 hours, it becomes obvious when your body is out of glycogen.
Ketosis is not some magical mind state.
When all glucose and glycogen reserves are exhausted, body will start producing energy by burning fat reserves. To do so it needs to produce ketones. When low or glucose body will gradually start switching to ketosis to supplement energy needs and eventually fully switching to ketosis and only way of producing energy.
When glucose is available it will be used as primary 'fuel' source again.
That is a reason why after fasting its important to ease in the cards and sugars.
Coming up from a fast and eating a lot of food quickly may cause refeeding syndrome - regardless of whether it is carbs or protein/fat.
Switching back to carbs from pure fat or pure protein is not dangerous in the same way, AFAIK (but I'm not a doctor).
I use ketosis strips (Ketostix). You pee on them, and they tell you. Totally worth the 5$ they cost.
I'm happy in full ketosis, or no ketosis at all, but the "half-way" ketosis state is just pure torture.
The strips let me know how good I'm doing, so if things deviate a bit, e.g. because one day I ate ""too many carbs"", I notice quickly, and auto-correct.
On top of that, your body gets more efficient at using ketones as an energy source over time so your urine ‘overflow’ can spike at various points just because your body consumes less ketones for it’s energy needs. All of that is latent knowledge from random reading over the last 4 or so years, so grain of salt and all that.