I am a keto’er, so I would like to point out my perspective may be a bit biased. However, I am doing keto as en experiment, it actually is the continuation of my soylent experiment.
The underlying issue in these discussions (keywords: keto, grain brain, carbohydrate and, ultimately, fat) is that the USDA recommendation (or equivalent of your area) puts a lot of emphasis on carbohydrates. A lot of people can live very healthy on a high carb diet (high carb being the USDA’s recommended 300 gram of carbs), but a different lot of people would fare better with a slightly lower amount. This heigt of this amount generally seperates the different parties in these discussions!
The recommendation of 300 gram carbs per day is not scientifically established (source: ‘Good calories, bad calories’, by G Taubes). This recommendation is based on the scientific knowledge of the fifties and sixties (1), mostly that fat “causes” heart attacks and heart attacks should be avoided at all costs. Not only is this knowledge proven incorrect in the 50 years since, but heart attack preventing diets do not lengthen the average life span: the odds of dying from certain conditions changed (cardiovascular disease decreased, cancer increased), but the overall life expectancy remained unchanged.
Next, a high intake of carbohydrates results in a high insulin response, which prevents the body from burning fats. This is the reason why a large quantity of fast carbs is bad for you: you get so much insulin in your system, that the sugar in your blood is processed (burned or stored) long before your insulin levels return to a rest state. Approximately, sugars are processed in 2 hours, while a large amount of insulin can take 4 hours to return to normal levels. This means that in these 2 hours, you are hungry even though you consumed enough calories!
A good debate discusses what levels of carbohydrates and fat are acceptable (2), and how carbs should be spread over the day. Unfortunately, most discussions concern parties trying to convince eachother that fat/carbs are bad.
Tl;dr: are carbs bad? It depends on your body type, but I (and others with me) believe that the USDA’s recommendation of 300 gram of carbs is not the optimal amount for the average population, since it was established to limit fat intake.
(1) The subject was young enough in these times, that long term studies had not been sufficiently done (i.e. they were too short, too few subject, or they didn’t actually investigate the issue of macronutrient balance).
(2) Notice that protein, the third macronutrient, is not considered. This is because we have quite a good understanding of the roles proteins play in our metabolism.