The problem is that no spell is guaranteed to work 100% of the time.
First one must consider the knowledge and experience of the person casting the spell. If you fully comprehend the necessity for focus and intent and are capable of raising and directing energy towards your specific goal then there is more likelihood that the spell will work. If you aren't then it is likely that no spells will work for you.
Yet even the most experienced and talented spellcaster can have spells go astray. Your health, your mental state, outside distractions, etc can have a negative effect on whether a spell works or not.
Sometimes the Universe or the Gods (take your pick) simply decide that a spell is not going to work and something goes wrong.
And of course there are those spells on SOM, such as ones in the Fantasy and Trick sections, which will never work no matter how talented and experienced you might be.
So if you pay a bunch of money to someone there is still the likelihood that the spell won't work. And if they try to guarantee that every spell they cast works, then they are simply not telling the truth and should be avoided.