1) You'll be losing muscle before you lose fat
2) You'll be lethargic which can result in no weigth loss because your calorie expenditure will match your (reduced) intake
Overcoming these two issues is what most slimming/fitness regiments are about.
Here are just 3 studies, but there are many many more.
https://www.ncbi.nlm.nih.gov/pubmed/24065788
2. You would only really be lethargic for a small amount of time as your body adjusts to blood sugar levels. It's important to note that people who are losing weight are not low on energy because they are lacking calories. They have a ton of calories, they just aren't very good at using the fat to maintain blood sugar (yet). This should be roughly a 1-4 week transition, and again exercise will help a lot with it.
I wish it was that simple. From the studies I've read and personal experience, the human body stops using fat as its source of energy once it goes below certain percentage (the lower the percentage, the harder it is to make it use fat). It's pure muscle catabolism and low levels of energy from there on, unless you use tricks like intermittent fasting and macro cycling (and resistance training).
The deal is whenever you lose weight you lose muscle and fat. All you can do is shift the ratio. Resistance exercise helps shift that ratio. Proper diet (ie: enough protein) helps shift that ratio.
2) is one of the reasons that coffee exists on this earth :-)
Lost 50 kg doing CICO. To keep your muscle, watch your macros and exercise.
Losing weight is actually pretty simple. But I won't argue that it is hard.
It depends on the genetics, but yes, if you're really overweight, losing lots of weight is generally pretty simple. At some point, though, you'll hit a roadblock and that's when those nuances can become important.