All dental materials (including composites) to be used in patients' mouths in Singapore have to be licensed by the Health Sciences Authority of Singapore (HSA). These materials can only be purchased by a dentist registered with the Dental Council of Singapore or a dental clinic licensed in Singapore.
Beauty salons and beauty therapists DO NOT HOLD ANY OF THE RELEVANT LICENCES to purchase dental materials and equipment legally from the official dental supply companies in Singapore. Most likely, they have purchased them online through websites like eBay or AliBaba. Many of these suppliers are based in China.
Since the materials and equipment used in these beauty salons have not been verified by HSA, or perhaps even other regulatory bodies like CE (in the European Union) or the USFDA (in the United States), we have no way of knowing if it is genuine and has been tested for safety. You are essentially at the mercy of the salon operator and the beauty therapist, especially since most patients are not aware of the different types of materials available for veneers.
There is no standardized training for beauty therapists offering composite veneers. Many have attended only a single session often conducted by another beauty therapist. I am not sure what is covered during the training session, whether any relevant dental anatomy, oral biology or troubleshooting is included.
In order to adhere the composite material to your teeth, the enamel surface has to be cleaned and treated with an acid gel (35% hydrophosphoric acid). The acid treated surface (etched) is impregnated with a bonding resin that has to be set using a special LED curing light. Only after these 2 steps can the composite be applied to the tooth surface and sculpted. The composite also must be set using the same curing light.
If NO tooth preparation (shaving) is done beforehand, the final veneers can be quite bulky. This may not be a desirable look for some people.
If no care and attention is paid to shaping the interdental (between the teeth) and palatal (the back surface) contours of the veneers, unhygienic plaque traps can form. Some beauticians may also join all the front teeth together when placing the veneers since this is much easier to do than having individual veneers side by side. Since it will be practically impossible to clean in between your teeth with floss, accumulation of food debris and oral bacteria will cause swollen bleeding gums (gingivitis), bad breath and tooth decay (over the long term). I do not think that beauticians will know how to manage these conditions that dentists see and manage daily. The longer the veneers are left on your teeth, the higher the chances of you developing irreversible damage to your teeth, gums and even underlying jawbone. In that sense, composite veneers are not completely risk-free and reversible.
Should you wish to remove the veneers, your dentist can do it safely without harming your enamel but it is a long and difficult process. There may be a significant fee for the clinical time required to do this safely.
In the end, if you should choose to have veneers done with a beauty salon, please remember that dentists as health professionals are bound by law and by ethics to DO NO HARM. The same cannot be said for a beautician trying to do the work of a licenced competent and ethical dentist.