Is the relative weakness of post-hypnotic suggestions also due to the fact that they do not necessarily become beliefs, or actually conflict with previously held beliefs and get discarded?

In actuality, both things can happen. This is why people cannot be commanded to be different than their true nature, and why this approach cannot be used for a therapeutic transformation to overcome personal liabilities and inclinations even when highly desired consciously by the individual. There can be temporary compliance to post-hypnotic suggestions and occasionally, with the right match of instructions to inner willingness and desire for something to happen of a different nature, the suggestion will end up being acted on and will seem magical, but that is simply bringing in a little more internal firepower to add to the willpower of the subject. But this cannot be relied on in a routine way, especially when there is any kind of interior resistance to change, and that is typically the starting point for any kind of therapeutic need and the reason why it exists, that the person themselves is powerless to transform their own actions, thoughts, and feelings to achieve a better end result for a particular issue. That helplessness to solve their dilemma is in reaction to inner conflict and constraints imposed by conflicting beliefs, in most cases, in addition to a good deal of learned fear and other negative emotions highly associated with a certain situation or undertaking so it will be avoided, many times at all costs, and so will post-hypnotic suggestions to overcome such resistance—it will be difficult by the subject, if not impossible. Post-hypnotic suggestions are merely recollections, and when recalled can be acted on as though someone is beside them making a suggestion for a particular action to happen. It will most likely be enacted as long as there is no inner resistance to doing so, but that will fade in time. And if the post-hypnotic suggestion is an attempt to change something quite fundamental about how the person sees their place in the world, and their self-image, and conflicts with their own expectations and comfort level, if it clashes with inner fears about the hazards of what is being suggested, the post-hypnotic suggestion will likely be ignored and ineffectual. In fact, any suggestion that goes against already established beliefs will have little or no influence, and this will not be readily observable to the hypnotist because many of the beliefs are uncertain and might be held within the subconscious level of the mind and not consciously. So what seems quite possible and eagerly embraced by the conscious mind of the subject, who might dearly want to change something about themselves, simply has no idea of inner beliefs to the contrary that make change extremely difficult to happen because the well-entrenched beliefs will block any such notion. In such a situation, the only thing that can break the logjam is doing some belief work, to change the beliefs in residence, to open up the possibility of acting differently than the inner programming dictates.