The major problem that hairballs can create is intestinal blockage. Sometimes, the cat can neither pass the hair in the stool, nor vomit up the hairball. The hairball remains in the cat's intestines, partially or even totally blocking them. Blocked cats exhibit signs of constant attempts to vomit, often perceived as a dry cough or retching. Constipation can also indicate blockage, and it's often accompanied by loss of appetite.
Older cats are also prone to hair-induced blockage.
As the cat ages, its digestive system becomes weaker and its bowel movements may not be strong enough to expel the hair. Blockage can be a serious medical condition. Other things than hairballs may cause intestinal blockage, so you should contact the vet as soon as possible for a professional diagnosis and treatment. This is probably the best way to prevent hairball problems. If your cat suffers from hairballs despite constant brushing, you may consider using a hairball prevention product such as Petromalt.