Problems in human sciences

Oxford Dictionaries defines human as "relating to or characteristic of humankind". Then, it defines humankind as "Human beings considered collectively". This is a nice illustration of the first and main issue with using science on humans, which is that the term "human" is still not clearly defined and still changes. Of course, you can define "human" in that they belong to the Homo Sapiens species, however, it should not be ignored that even this was not accepted a couple of centuries ago.
Closely related to this is the issue that we still aren't entirely clear on what makes up a human - the composition of a human. Biology and its several subgenres are trying to figure that out, the smaller parts being left to chemistry. However, there are always new discoveries in the field. One previously thought part of the human is crossed out, it's replaced by another one. And let us not forget of the grand frienemy of science: religion. Naturally, the parts of humans that cannot yet be explained are attributed to the One Above, or the Soul.

Another limit that science just can't quite overcome is perspective. As the phrase goes, "one man's trash is another man's treasure". Someone with middle-class income in one country might be considered wealthy in another or borderline under-privileged in a third. Death for the hunted means life for the hunter. 

As everyone knows, everyone is different and special and beautiful in their own way and whatnot, which creates yet another problem for science. Because the aim of science is to create laws applicable to everyone and everything (such as gravity, everyone has gravity), this is made harder by the special sunflowers of this world, who get offended if they have something in common with someone else, or by those who advocate the difference. Of course it must be recognized that it's true that people, groups of people, and cultures are different in one way or another, but it should also be highlighted that on a grand scale of things (if we look from far enough), the differences are really minor and we actually have much more in common (there was this small German guy with a funny voice and a strange moustache once, who really wanted to emphasize the difference his nation had with others).

The last main problem in human science I will mention now is ethics. Science uses experiments. Experiments hurt their participants. Ethics try to intervene at these points and not let people hurt other people. They created very strict guidelines. Then someone discovered that if we can't hurt people but we have to experiment, we could hurt animals. And this is the story of how animal experiments were created. Of course they also have their own guidelines, but they aren't that strict. And besides, the media doesn't give animals as much attention anyways. Consequently, not as many people care about animals. And therefore, animals are harmed much more in the experiments than humans. But luckily, we have animal rights groups now. They will help. Hopefully.