Well, it may be that they are "each and all" teaching perfectly fine
Shikantaza Zazen. Let me explain ... I used to think that there is only "one way", so everybody else must be wrong. Most teachers sure act like there is only "one way" (which, no coincidence, is always the way they happen to advocate).
But I do not think that there is only "one way of
Shikantaza" any more.
...
The bottom line and true key (for me anyway) is that
Shikantaza Zazen involves some form of sitting, placing the mind on some object of concentration, dropping judgments, dropping divisive thoughts of this and that, allowing emotions to settle and fade, thereby attaining a sense of balance. The body and mind settle, the breath settles, calm, peace, wholeness is tasted ... the world appears different from before, less broken into pieces, all of one piece, without pieces, Peace beyond pieces, etc. ... . (For want of a better term, let's call this result of
Shikantaza as "THERE")
I am sure that almost everyone (with experience) is experiencing something like that when they sit, at least some of the time, and knows THERE.
Now, for example, one teacher may associate his "getting THERE" in his mind with needing to be in the lotus posture focused on the spine (because that is what he does to bring about the above, so he assumes Lotus Posture + Spine = THERE). Another teacher may note that focusing on the breath leads to THERE.
Another teacher (this is what I, Jundo, teach) may emphasize sitting with the mind "
Open and Spacious, focused on all objects and no particular object, while letting thoughts drift from mind". That also leads to THERE.
Everybody's right! Everybody gets THERE!

Really just a matter of different places to place the mind, like different ways to sit on the bicycle seat.
Of course, yes, you can debate the pros and cons, merits and demerits of each way of getting THERE. For example, I emphasize "open, spacious sitting" because I personally believe it makes this practice easier to take off the Zafu, into the rest of life. I also teach that one need not be sitting in Lotus Posture for this, but any balanced way of sitting (in fact, I encourage folks to practice
Shikantaza all through their day ... standing in the grocery line, on a crowded subway). I personally believe that it leads to an ability to develop "thinking not thinking = non thinking" as Dogen described. I find this comes more easily from sitting with "
open spacious mind, eyes not closed, fully present with one's surroundings yet focused on everything-and-nothing-in-particular, dropping the judgments, letting thoughts come and go, finding again and again the quiet space between etc." ... which makes it easier to bring the "lessons" of Zazen off the Zafu into our day-to-day lives (cause we can't be focused on our spines all the time!). That also brings about "softening and losing the sense of self", as do the other methods, but by not limiting focus to the posture, it is easier to be "at one" with the whole world and all of flowering life, and to bring it off the Zafu ... which I believe was Dogen's and most other Zen teacher's main point.
But that is just my
Schtick, my way of teaching THERE. I do not think the other ways are necessarily wrong.
Let me close with one last observation: Some folks are more into the body, some more into the head (now, mind-body are not two, but they are two). For example, I have encountered folks like Nishijima Roshi (who got into Zen as a runner in track in high school and often compares that to Zazen), various martial artists, ballet dancers and others who are more into the "losing the self" through the body.
Me, I happen to emphasize the mental side, and I personally think Dogen emphasized the mental game of
Shikantaza more than the physical side (because my reading of his writings emphasizes that he was conveying in words the mental experience of how he was tasting the world through his Zazen ... he rarely spoke of the physical manifestations, let alone spines and breathing). But, again, that is just my reading (and to repeat ... mind-body not two).
BOTTOM LINE: Many ways to screw in a lightbulb. Many a good teacher will get you THERE ... which is everywhere, and no place at all.
Gassho, Jundo