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. However, 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, there is still a certain need for cognitive responsibility. This is despite the ability of hypnosis to empower the brain. Very often hypnotherapists cite alterations to the subconscious mind, implying that you are merely a participant in the process. A natural problem to this view though is that we are uniquely conscious beings at some level! Much of this consciousness, makes you feel like you made a conscious choice or decision. However, it is worth remembering, that all behaviour, thought and words are processed in the brain moments before they become a physical manifestation? The downside for us is the dialogue that takes place after the subconscious processing (intent) and the conscious realisations that follow (outcome)! Hypnotherapy merely allows us to have a better perspective at the conscious level, much of which can occur out of conscious awareness. What? We are conscious beings but we are not always consciously aware of that and that is the place where issues develop, fester and manifest! Hypnotherapy is an art and science and practiced often enough (self-hypnosis), combined with cognitive responsibility, can lead 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 simple, when the brain repetitively fires on the same experience, over and over (real or imagined), synaptic strength (long term potentiation LTP) is achieved. 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!. During a potential LTP situation, there is less resistance from the postsynaptic neuron. This is a consequence of an event called an action potential. Essentially an action potential is the initiation phase of a neurochemical messenger process, aka neural communication! Over time the repetitive nature of action potentials/LTP creates what we call habits or behaviours and, as you know, practice makes perfect! In essence, our brain is 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. This occurs through sensory modalities, e.g. during the conscious fact-finding part of the session, then through the auditory cortex during the hypnosis. This is often what other hypnotherapists refer to as effecting change through the subconscious mind! But in reality, 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 at some things than others! It is because of the uniqueness of each us that guarantees cannot be offered. This is because we (the client) are the very ones that ultimately (subconsciously) determine the outcome!