Truism is referring to some statement that is clearly true and evident, it often sounds foolish and it is used in situation of irony. Since it is evident, it don't need evidence or description.
Examples of truism that we see often:
The apple never falls far from the tree.
An apple a day keeps the doctor away.
A friend in need is a friend indeed.
Life is not fair.
I will give you an example of ''Life is not fair'' truism.
This truism is worth defending because in some situation it is not considered as truism because it is up to people and ideas that they are having about life. It can be true or false. It is challenging because it depend on people and their situations or positions.
My experience is considering that this truism is not worth saying because life has numerous of situations for us and we are not always seeing some fair things that are happening. It always can be worst and if we always repeating that ''Life is not fair'' it will give us negative emotions which is not good.