I read that 要するに and つまり are not always interchangeable. Still, I don't really understand the difference between the two...
The two can be interchangeable but where as 要するに means "To summarise" or "in short". つまり is usually taken to mean "in other words":
It was a big defeat. In short, there was big difference (in strength).
I have lost my wallet. In other words, I have no money.
There is a similar word to つまり、すなわち which also means "in other words" but is not interchangeable because it does not convey a a conclusion:
母の兄、 すなわち おじさん。| My mother's elder brother. In other words my uncle.
I explain this because (I think*) in the following sentence つまり is interchangeable with either 要するに or すなわち, illustrating the nuance you are asking about.
I understand my friend's work is dangerous, dirty and hard. This kind of work, incidently is commonly referred to as 3Ks work. In short the ３ "ki"s or "K"s often come together.
*I should be grateful for if others would confirm correct me on this.