This question probes the difference between poses that require opening in the hip complex and poses that deliver opening in the hip complex. Unfortunately, when we are not appropriately trained, either as students or as teachers, the tendency is to lump all poses which involve the hip into the category of “hip openers”.
For example, the pose called pigeon (eka pada raja kapotasana) is commonly put forth by teachers as a hip opening pose. And while that is not false, the degree of opening [I]required[/I] so far exceeds the amount of hip opening [I]facilitated[/I] that it is, to say the least, misleading to call it a “hip opener”.
Additionally, different expressions of Yoga have a different priority system as it relates to safety in the practice (of asana, since that is the topic). Some practices are very, very safe and only slightly effective. Other practices are incredibly effective but radically unsafe - and all points in between. The mission of a sound teacher is to find a way to provide the student with the most effect in the safest way possible.
For opening the hip complex it is important to understand that there are eight movements, actions or ways the hip joint can “behave”. When we spend much of our active lives only working flexion and extension (as is the case in cycling, running, jogging) then there are quite obviously 6 other actions that are neglected. One of those is compression and since the hip lives in compression as a result of being bipedal there are 5 remaining movements. So a sound hip opening series would attend to those five.
Only working external rotation and abduction, as is the case in Eka Pada Rajakapotasana, Baddha Konasana, Ardha Padmasana, Vrksasana, and Janu Sirsasana would be a practice that creates imbalance in the hip complex. However, they may contribute to the practice of Padmasana and frankly some of these poses are safer for the knee joint than padmasana is, especially for neophytes.
To reduce strain on the lower back it is best to explore a hip series in the supine position rather than seated or standing.