Yes, it applies even in Kali yuga. We have to remember, however, that nishkama karma is an yoga and most probably cannot be practiced by an ordinary person.
If you are attached to the result of your Karma, then you will get easily discouraged if you do not attain your goal. If you attain success then there is of course no problem! All philosophy is about failure.
You are asking about the practicality of nishkama Karma which is part of Karma Yoga. A person practices Karma Yoga when his work benefits other people and the work is done without caring about the result or name, fame, promotion, wealth etc. This type of work is called nishkama karma. Nishkama Karma is not about the ordinary karma like professional work done by us. The idea behind nishkama karma is that God dwelling in the other person is giving us a chance to serve God and thus help ourselves. Whether the other person is helped will depend on God.
Does detachment from results mean we should not plan at all to get good results when performing karma ? Should we not focus on efficiency of work because we do not care about results ?
You should of course plan to the best of your ability. You also should focus on efficiency of work. You should, however, always remember that it is hubris to think that one human can help another human.
Only Ishvara can help
Therefore, O dear one! Give up your infatuation born of ignorance,
which makes you feel worried, thinking – how will these helpless
people get without me? To think that one can save or protect another
while one’s own body, subject to the power of time, karma and Guna, is
decaying, is like a person in the grip of a python thinking of saving
Srimad Bhagavata Purana I.13.44-45
An example of nishkama karma is the Sun. I am posting a poem by Hafiz, the 13th century Persian poet, that captures the essence of Karma Yoga.
All this time
The sun never says to the earth,
what happens With a love like that,
It lights the
The Gift, Poems of Hafiz, The Great Sufi Master, translated by Daniel Ladinsky.
The sun does not care what man does with the energy of the sun that makes life possible on earth. The sun just keeps on pouring energy. It doesn't matter what field you are in. If you can help even one person without caring about the result of your help then you are doing Karma Yoga.
It is not possible to do karma Yoga in a job setting. You will most likely get fired if you are unattached to the result of your work. Karma Yoga is after all a Yoga and unattached work can only be done as part of divine work.
A householder will find it difficult to do nishkama karma. He will hanker after the fruits of his work. He will certainly lose motivation if he doesn't have the expectation of good things of life and if his effort fails. It takes a great deal of effort to work with passion and yet remain detached from all expectations. Most persons only do karma. Only spiritual seekers try to do karma Yoga. There is no doubt that Karma Yoga is very hard.