There are lots of people who will tell you its unhealthy but to be honest it's rare that they are doing it properly.
Keto for me resolved health problems , I've lost just under 7 stone now , sleep better , have massive energy , it stopped migraines and stomach issues. It massively helped my mental health and resolved my period issues.
But ....its not a quick fix. Calorie restriction can really damage your body and after years of doing that I needed to resolve my health issues. It's not a diet and if you treat it as such , pp are right you will feel like crap. Cheat days are silly as they make you feel I'll.
Most people who comment about how they felt crap did it for a couple of months and gave up, you do have to see it through. Initially you will feel a bit crap because your body is adjusting to using fat as a source. If you see that bit through the effect is amazing. My blood pressure normalized for the first time in years , my cholesterol is dead on.
I eat more calories now than I ever have and I'm still losing weight.
Research the science behind it . Be clear what your body is doing and be prepared for stalls. It's hard to explain but you get this odd clarity , your mind is sharper and it shows you exactly what carbs and insulin resistance do to you.
However again, it is not a quick fix, if people do it as a diet (I'm not saying you will but some of the above negatives clearly did) then no you will feel I'll and it wont be sustainable)
For me , it gave me my life back.