This is a broad generalization.
The same thing can be said about any diet that you use to lose weight and then stop and go back to old habits without learning anything.
Just because Keto causes you to "lose weight fast" it's bad?
I've lost over 100lbs over the last 2 years doing Keto and exercising. It's fuckin' annoying hearing people tell me it's a bad idea over and over.
The biggest criticism I see about Keto is that you "often gain the weight back". That's any diet.
uh.. what? lol
The opposite is true actually. I hate bringing it up because people are completely ignorant about it.
Because Keto isn't going to stop habits like:
Over Eating
Managing intake of macros (i.e. not eating shit tons of carbs)
Creating a balanced diet
Cooking a balanced diet
How macros impact or compliment a workout plan
Again, it's not bad that you lost weight, but you shouldn't convince yourself that you've gained all the tools necessary to at any point abandon the diet and maintain your current weight. Furthermore, what I am a proponent for isn't a diet. It is simply understanding the food that you are consuming and how it impacts your body. Keto teaches you what NOT to eat, but it doesn't teach you how you can balance those things to achieve results or lead a healthy lifestyle. I'm not suggesting that Keto is unhealthy, but I am suggesting that most people on keto have almost no idea what it would mean to leave keto and balance their diet accordingly.
Again: Keto is good for very specific uses and scenarios, but it should not be a lifestyle for anyone. Another problem being people don't like to be told they've maybe made a mistake or could potentially be "wrong" about something. No one who has seen great results on Keto is going to accept that there is still a lot of ground that they never covered on their path to weight loss, and that their results could have been achieved in a more sustainable manner, albeit requiring a bit more work. So I completely understand why the idea that keto is bad as a long term lifestyle really pisses a lot of people off, because in the short term it gave them something they didn't think they could have.
If all you've sacrificed to lose weight is your bread and pasta, then what have you taught yourself? What understanding do you have of your body? Nutrition starts with education and work, not by asking for a buger with no bun.