If you read his famous "Autobiography of a Yogi," or study his lessons or other writings, it is more than obvious that devotion and concentration are a part of it. Still, it is my view that it can be very helpful to students learning raja yoga, including kriya yoga, to focus on the energetics of these techniques and of the stillness that follows their practice.
I say this because the other two aspects are not dominant characteristics in our society at this time nor, very likely, for centuries to come, given the line of development of consciousness that we observe in the 20th and early 21st century.
An age of personal liberties and knowledge, and of democracy, is hardly inclined to traditional expressions of "hierarchical" devotion. I won't go as far as to suggest we only teach kriya as only a science the way Transcendental Meditation became popular, but I do wish to note that Yogananda's first book was called "The Science of Religion."
Note, further, however, that it was also called "religion." The motivation that it takes to meditate deeply suggests, hints, and indeed requires, a sensitivity of feeling and refinement of consciousness whose receptivity contains the seeds of a devotional nature.
As to mental concentration and power, we live in an overstimulated age subject to an ever-increasing pace of outward activities and change. This age is not conducive to developing strong mental focus. Indeed, the biggest plague of our times is loss of memory and loss of focus as illustrated by preoccupation with cell phones and similar devices. This trend is far, very far, from slowing, what to mention reversing.
Maybe one's mind is scattered when trying to meditate. Maybe one is not feeling particularly inspired. Instead of being discouraged by one's lack of concentration or devotion, try focusing on the energy of, first, the physical body (using, say, yoga postures or Yogananda's Energization Exercises); and, then, as you begin your meditation practices, focus on the subtle life force in the body. This karma yoga approach is a fairly neutral and relatively easy feature of the experience of meditation. The core techniques taught by Yogananda ("Hong Sau," "Aum," and "Kriya") all fit neatly within this framework.
Yogananda, in Chapter 26 of "Autobiography of a Yogi," that the word kriya has the same root as the word karma. The meaning here is, simply, action. Thus we see a strong hint of the relationship of karma yoga to kriya. Swami Kriyananda, direct disciple of Yogananda and founder of Ananda, often pointed out in his lectures on the Bhagavad Gita that the "action" described in that great scripture includes the action of meditation.
In that scripture, Krishna says to Arjuna (us), that one cannot achieve the actionless state (of oneness) by refusing to act. Thus he hints that the practice of meditation requires specific (i.e. scientific) techniques. Elsewhere he describes the penultimate technique as "offering the inhaling breath into the exhaling breath." (Clearly a reference to kriya yoga and similar advanced techniques.)
I have found in my own meditations that focusing on the energetics of the meditation (from the body to the chakras to the subtle spine) affords a tangible focal point such that it leads to the stillness of breath and mind that is the initial goal and necessary first stage of meditation. To quote the 1972 Alka Seltzer commercial that made this line famous: "try it, you'll like it."
Blessings to all and happy meditating,