I am not a scientist, so I can't say for sure, but unless you are a very light person with a very large and strong horse, I would refrain from mounting a crunch. What I mean is that I would refrain (and not to say that everyone else should refrain as well), and I'll tell you why!
I look at Tam's obeisance, which he does quite beautifully now, and he does not (or perhaps cannot...I'm not sure) raise his back in that position to protect himself from the weight of a rider. As well, the front legs are not in a good position to support extra weight. Being stretched out in front, with the back essentially dropped, I would worry about straining something either in the front legs or the shoulders or the back.
When Tam bows (a one knee bow, with his head up), his back is arched upward slightly, and he's firmly supported on his knee, and there is a shoulder-supporting straight line described by his foreleg. The other leg, is of course extended out in front is unable to act as a support. I have mounted Tam's one-knee bow a few times now, but it is rare that I do it, and I do not use it as a means to mount every time. It is not the best weight supporting position, but I think it's somewhat better than an obeisance.
I do not think that I would even sit an obeisance (ask Tam to do it while I was in the saddle) for the same reasons as above...at least not now. Perhaps when he's ten years old or so...and if he is very strong in his body, completely healthy and completely finished growing. But while he is still possibly growing, I will continue to be extremely careful how much stress I would put on his joints and tendons, etc. But to me, sitting an obeisance seems too close to sitting a horse with a dropped back in any movement...and with a dropped back, the horse cannot protect himself from the weight of a rider.
So I'm only thinking about cautioning here...I don't know the relative sizes of everyone...and if you have a small child with a stocky and well built pony, then I don't know that any harm could come of it - rather, it's just something to be mindful of so that one can be aware to watch for hesitation or any kind of objection from the horse...they will tell you if it's uncomfortable if they have been allowed to have that kind of voice.