What is the difficulty with cloning and genetic engineering in humans?
The question you are asking is an ethical problem rather than a biological one. This particular nature article highlights the problem I am speaking about. 
In brief, children afflicted with SCID were subjected to what at the time was termed as a pioneering retroviral treatment. The idea being you replace the defective gene with the real one, in case of the quoted study, of 11 patients, 9 were cured and one of these babies developed leukemia due to the treatment. This was one of the first trials and it showed how these treatments can go awry. 

Fast forward to today, last year you may have heard, something called the three parent babies garnered a lot of attention. This is basically mitochondrial donation, where a third-party provides mitochondria for IVFs for reasons where parents opting for IVFs may not be able to undergo successful IVFs due to mitochondrial mutations. This posed and still poses a serious ethical question, because what you are doing is mixing heritable DNA between lineages, please note that most geneticists will never cross this line, i.e. editing the genome such that the offspring carries the edited genome. Almost always the modified individuals are sterile (I know of no GMO crop which is not sterile)

But not to say, we have done nothing in terms of genetic engineering.

Scientists for the same reason (A human life is more important than any animal's) are wary of editing the human genome directly, so we now we also have something called siRNA treatment is just one example. And this is a list of siRNA based drugs which were in various stages of clinical trials in 2013. I know of a few which entered trial since then.

Lastly, this is just the tip of the iceberg, there's a whole lot which could be said on this topic, but it boils down to the same ethical problems.

Just so you know, since 2001, we have a lot of newer genome editing tools, such as TALENs and CRISPR-Cas. But remember, they still have the same problem as retroviruses from 2001, the issue about off-targets. 

What if your tool targets a region which it isn't supposed to target?

Will another child develop a condition due to the treatment?

To avoid such an issue, we need to understand much better how the genome works, and sadly still we don't know enough. I cannot quote the study here because I have no idea where it is published, but at a recent conference I heard a talk from a lady about how retroviral integration works. The idea being, if retroviruses don't need specific integration sites, they should integrate at the first genomic region they encounter, that is not the case. I will not go into more detail. But I hope you have some food for thought.

UPDATE ON Genetic Engineering at a preconception stage



This is a link to the Nature article on Venter's minimal cell. There also exists an artificial Yeast genome page. The point I am trying to make is, we are really at the basic stages even now.

I will outline to you the behemoth scale problem we are facing.

2002: The first human genome draft is published. All the media is rife with stories about how we are going to unlock the secret to life.

Since then, DNA methylation and histone modifications have come to the foray as regulators of genome functionality. 

We knew that DNA was transcribed into RNA and translated into protein. Noncoding RNAs came to the foray and messed up everything. 

We thought that DNA functionality was governed by DNA methylation and Histone modifications, come 2009, we saw the first major article on Chromatin architecture. We understood that the genome has switches called Enhancers and promoters which regulated when and for how long RNA is transcribed. But sometimes these switches were so far away, we did not know how they switched on their target genes.

The problem can be summarized as this, making an artificial bacteria is easier than a human. In humans you have multiple layers of regulation, there's the DNA, where one part is silent (heterochromatin) and one part is active (Euchromatin), DNA methylation and histone modifications affect how genes are expressed, these modifications allow opening of the DNA so that genes can be expressed, The DNA is further compartmentalised (architecture) so that switches only affect nearby genes in 3D space. Non-coding RNAs act as messenger in such areas relaying proteins to their target DNA regions. So the cell is a mess!

Not to say that goat biology is simpler, just that a goat's life is cheaper than a human's. So we wouldn't feel a thing while "euthanising" goats for the advancement of mankind.