So what's the point of your post? Are you looking for advice? Do you feel you fall into one of these categories? If so, what are you planning on doing about it?
You should read this article by @Chris_Colucci. Seriously, read the whole thing.
Regardless, I disagree with you 100%, for the simple reason that unless someone has an underlying medical issue, you can't argue with the science of weight gain or weight loss. People who feel they have these problems simply don't take the time to learn enough about their body and what type of nutrition works for them.
Regarding the "hard gainers" who can't gain weight, I call shenanigans on you and WISH I had that problem lol. If you want to gain weight, eat more. Literally, it's science and extremely simple. If you feel you're eating a lot, and not gaining weight, obviously you're not eating enough, or you'd be gaining weight. 9.9 times out of 10 when someone tells me they can't gain weight (usually a very skinny person), but they just eat a TON, I ask them to write down everything they eat for a day. You know what? It's not enough, ever, otherwise they'd be gaining weight. You may need to eat 4,000 calories a day to gain, but if you feel you "eat so much" and aren't gaining weight, I will guarantee you you're not tracking your macros, calories, or eating as much as you think you are, otherwise you'd be gaining weight. There's no such thing as someone who can eat an infinite amount of calories and not gain weight. Let's move past this whole "I can't gain no matter what I eat" mentality.
Regarding those who "gain weight no matter what", again it's just a matter of understanding your body. These people typically have naturally slower metabolisms (like myself), lead a sedentary lifestyle, and eat more than they think they do. So, these folks should hit the weights harder and more frequently, will have to incorporate more cardio than someone with a faster metabolism, and be more strict on their diet.
Additionally, the same overall caloric intake with a different macronutrient breakdown might have dramatically different results. For me, I've found if I have moderate carbs and moderate fats, I'm more likely to gain adipose fat. So, I keep protein steady, and on days when carbs are higher I lower fats, and vice versa.
I certainly don't have a naturally fast metabolism and am prone to gaining weight easily if I'm not very strict with my diet, but I've taken the time to figure myself out, REALLY figure myself out, which mostly came with my recent contest prep, logging weight every single day for 9 months, and slowly tweaking my nutrition until I found the right formula for me, and I'm still learning and always will be. Anyone who believes they have an unsolvable nutrition problem just hasn't taken the time to learn how to eat properly and work on themselves.