Normally, Lumineers should not cause halitosis and/or gum disease. Both of these conditions are caused by bacteria in the mouth. Sometimes, restorative treatment can make it more challenging for patients to properly clean their mouths, causing an excess of bacterial buildup. If, for example, the shape of the patients' teeth changed dramatically (the teeth were made more bulky, for example), this could affect how well the patient could clean their teeth. When there is any change in the shape and size of teeth, it's vital that the patient be coached on how to care for them as it may require more work than what they were doing prior to treatment. Regular dental checkups are also very important. Lastly, there could have been an underlying bacterial buildup prior to treatment that was so mild that it missed detection.
Breakage should be a minimal problem with Lumineers. While they are in general very thin, they are bonded to the teeth and this reinforces them so they are really like a new layer of enamel on the tooth. If the bite is checked and managed and the bonding process is done properly, Lumineers are just as strong as conventionally prepared porcelain veneers. The thing to remember is that Lumineers is a trademarked name of a type of porcelain veneer (like Kleenex is a type of tissue). There are many different types of porcelain veneers out there. The brand the dentist chooses is less important than how the teeth are prepared and treated before, during and after the veneers are bonded.
The best way to avoid problems is to choose the right dentist. It's important to ask your dentist how much post graduate training he/she has in advanced cosmetic dentistry. Some questions to ask are: has he/she taken many courses, were they hands-on courses, how long have they been practicing this type of dentistry, how many cases have they done, how complex were the cases, etc. Not all cosmetic dentists are the same.