The guarantee is, the more you apply your mind, the better it works; that's a promise
However, to answer your question frankly, the answer is no. That said, anyone who says they can guarantee results may be stretching reality beyond its natural bounds. As it would be with a Doctor, the best I can give you are the odds of probability. The same goes for medications, there are strong odds in favour of most medications being effective but all that can be offered, by way of guarantee, are clinically proven results; statistics of probability. Over the years, empirically speaking, I have found hypnotherapy to offer each client an outstanding prospect of success. However, even when using hypnosis, despite its ability to empower the brain, which in the context of hypnotherapy, is via what we call the subconscious mind, there is still a certain need for cognitive responsibility. This simply means you are a participant in the process. Much of this participation will occur at levels below consciousness, although it often feels like you made a conscious choice or decision it is worth remembering, that all behaviour, thought and words are processed in the brain moments before they become a physical manifestation? Practised often enough, self-hypnosis, combined with cognitive responsibility, leads to pronounced behavioural change.
The change that occurs in hypnosis is caused by the way your brain processes the information it receives, relative to the hypnotic prescription. It occurs in the same way that all brain functionality occurs; i.e. at a cellular level, the place where memories are stored, activated and expressed. There is actually no magic involved in the process. It's merely the case, that when the brain continues to fire on the same experience, over and over (real or imagined), synaptic strength (long term potentiation LTP) is achieved and similarly, this can cause the opposite effect (long term depression) in other areas - long term depression is a process of synaptic weakening and nothing to do with clinical depression!. At the same time, during a potential LTP, there is also less resistance from the postsynaptic neuron and that allows the process, called an action potential, to become automatic and this, over time, creates the LTP, which strengthens the response. In essence, it is like a well-oiled machine and what we term a routine or habit is the natural outcome of this process. However, this process works in both the negative as well as the positive! So, in a positive context, the secret to the effectiveness of hypnosis, is in the way information is delivered to the brain, and this occurs through many sensory modalities, during the conscious fact-finding part of the session and then, through the auditory cortex during the hypnotic part of it and this is what we often refer to as effecting a change through the subconscious mind! But . . . it is the brain's functioning that actually changes!
Finally, some clients work on themselves more than others and often get better outcomes. Some clients do absolutely nothing, other than attending the hypnotic sessions and still get outstanding results; others, usually very few, get absolutely no benefit. That is not necessarily because the hypnosis didn't work but perhaps because elements of their condition were not accessed or did not surface. Much less often, it is just the consequence of a mismatch between client and therapist; nothing other than a fact of life. Like anything in life, some people, be they plumbers, lawyers, accountants or doctors, are better than others, but irrespective of their personal standing; their discipline remains professionally intact!