Dark Lines Between Gums and Veneers - Receding Gums or Cheap Veneers?

What you have seen may not have been veneers, but old fashioned crowns that have metal substructures (called PFMs or porcelain fused to metal).  Some labs and dentists make them where there is a bit showing at the edges, and if gums recede they are seen.

If no metal is used, then if gums recede only tooth structure is seen.

Sometimes, if veneers were not done properly, fluids can stain the edges.  This would be very obvious after gum recession, but some stains can affect half of the veneer and not just the edges.  This usually means the veneers must be redone.

Usually if you see a dark line at the gumline on a patient who's had dental work done, it is a porcelain-metal crown.  Often the metal margin was buried below the gum tissue to hide it, but then over time it irritates the gums, makes them recede, and then the margin is noticeable.  Well done veneers have a margin above the gum tissue, making them easy to clean and they should blend into the natural tooth to be virtually undetectable.

The dark lines that you see around the edges of the veneers, or rather, most likely old dental crowns are from dentistry years ago. One of the advantages of porcelain veneers is that they can be made to match very well around the gum line, in fact almost all the time they are not visible at all. 

If you still have questions, then ask your cosmetic dentist to show you some of his before and after cosmetic dentistry photos. 

One of the goals of well done cosmetic dentistry is to make sure that you look great and that the result looks as good (or hopefully better!) than what you imagined.