Scent is a powerful thing. A single breath of CK One is enough to bring me back to being thirteen in spring -- I can hear my favorite song from back then, feel the cool, foggy air, and remember what it was when one of my biggest fears was entering high school next year. Smells are inextricably tied to memory and emotion in a way that's palpable and impossible to deny.
A lot of magick involves creating the right atmosphere. If you are in a situation where magick feels possible, it is possible. Creating this atmosphere takes some effort, sometimes even some pageantry, though what works differs from witch to witch. That said, there's a reason why so many spells and rituals call for incense and oils.
Scent is a very, very powerful thing.
Unfortunately (or fortunately, in some cases) not all of the ingredients called for in old recipes are available to us anymore. Some herbs are priced out of the reach of most people, some are hard to obtain ethically, and some animal-derived products (like civet or ambergris) are right out. Some people recommend using synthetic alternatives to illegal, unethical, or hard-to-obtain substances, but does this actually work?
I usually try to be a purist in my craft. There are a lot of things -- from stones, to oils, to paraffin candles -- that don't really do it for me when I'm trying to get into the right mindset. Many, though not all, synthetically-derived scents give me a headache or an uncomfortable burning feeling in my nose, so I avoid them. I don't like using petroleum byproducts when they can be avoided, so I stick to natural waxes and salves. I don't feel much when I handle man-made stones, pretty as they may be, so I don't use them. It takes extra expense and effort, but, for me, the end result is worth it.
My personal feelings don't really answer the heart of the question, though -- do synthetic ingredients work? Based on what I've experienced and read from other practitioners, the answer is a solid "Maybe."
When it comes to adopting a ritual mindset and creating atmosphere, synthetics certainly can do the job. Not everyone gets a headache when they smell floral perfume, after all. Besides, if these synthetic ingredients weren't good at evoking emotions, they'd never have hit the market.
On the other hand, when it comes to getting all of your energetic ducks in a row... Using synthetic ingredients can be a bit like herding cats. Say you want to do a money working, for example. You choose the right time of day for prosperity magick, the right day of the week, even the right point in the lunar cycle. You write a powerful chant to help you manifest your desire. You decorate your altar in green, and arrange it with appropriately-colored candles and curios. Lastly, you choose the right oils and incense.
At every step in this process, you pick things that align with your goal. A lot of those things may be dictated by your personal associations with them, others by your tradition or their history of use. When it comes to natural ingredients, there are more factors in play than personal association alone. Intent is a powerful thing -- the most important thing, even -- but it is never a bad idea to align your intent with your ingredients.
When you're dealing with a man-made object, whether a stone or a bottle of oil, you're dealing with something very different than whatever the synthetic is trying to mimic. This is not necessarily going to cause a working to fail (if it did, few practitioners would recommend using them!) but it does rob you of the opportunity to incorporate the energy of an ingredient closer to its natural state. This doesn't mean that synthetics don't have an energy of their own, but it's a bit like comparing glass and quartz, or imitation vanilla extract and a whole vanilla bean -- the sensory impact might be there, but there is a difference in depth and nuance.
Usually, there's a natural substitute for whatever the synthetic is trying to mimic. It's often said that rose can stand in for any flower, and rosemary for any herb. I can't think of any magickal purpose for which there is only a single, completely non-negotiable ingredient. There are even things that can take the place of blood, depending on what you're using it for.
Ultimately, it should be the goal of any magick practitioner to keep track of what they do and work toward refining their craft. Keep a journal, compare the effects of synthetics with natural ingredients, and see what differences exist for you.