Not sure who told you it was bad for you, I've predominantly heard it being touted as beneficial and longevity-increasing.
I've seen people losing 40kg (120kg to 80kg) by just walking and counting calories. It's the last 5-6 kg that are harder than the first 20kg is almost trivial to lose - one all the right habits are in place.
And habits are the hardest part. It's not the body, not laws of thermodynamics, it's just your habits (most of the time).
Obviously different people are different.
And it has utterly nothing to do with habits, you can see my other comment. It's about the whether you can even manage the other health consequences you suffer from your metabolism being so low.
Given lightweight excercise (walking,kettlebells, etc) that's almost impossible!
Yes, but at the cost of your immune system severely degrading so you suddenly get colds all the time. At the cost of extreme fatigue and brain fog. At least that's what's made weight loss difficult for me at times.
Because for some people (like me), your body drops its metabolism to well below what's healthy before it even thinks about burning fat. It's not any question at all of willpower or psychology, it's purely physiological.
That said, as someone who struggles with weight loss, it's still hard. There are both physioloical and psychological issues that make it challenging.
As an aside, keto is the only thing thats worked reliably for me so far - largely because of the caloric restriction it inherently forces. I stopped following it because people kept telling me they were concerned it was unhealthy, but I'm sure the weight is worse than anything else.