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, I have found hypnotherapy to offer each client an outstanding prospect of success. However, even when using hypnosis, despite its ability to empower the subconscious mind, there is still a certain need for cognitive responsibility. 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 the brain processes the information relative to the hypnotic prescription. There is actually no magic when the brain continues to fire on the same experience, over and over (real or imagined), synaptic strength is achieved. At the same time, there is also less resistance from the postsynaptic neuron and that allows the process (called an action potential) to become automatic, like a well-oiled routine or habit! The secret to the effectiveness of hypnosis, however, is in the way information is delivered to the brain through what we term the subconscious mind!
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!