I had my first set of veneers at 19 and my second set at 28. I only have my front top teeth veneered. Second set are potentially starting to cause issues 14 years later. I am dreading how much the next set will cost. Dentist has also said that there's only so many times I can replace veneers before I will have to move to crowns. The same dentist has done both sets.
I think I know what you mean by the black line round the gums and I've only seen that on people who have crowns, my veneers are porcelain and so are tooth coloured all the way up to the gum.
My veneers have comes with several issues.
First issue is I grind my teeth and have ground veneers off in my sleep, so consider if you are a teeth grinder or not. I have been lucky not to swallow them so have just had them re-cemented.
You have to be careful what you eat, I can't bite anything hard or tough like apples, chocolate or meat with my front teeth now and I was told not to eat or drink anything that could stain like red wine or tomato soup. Also plaque seems to be attracted to my veneers so I have to be really diligent in keeping those teeth extra clean.
My second set of veneers were made a bit longer than my teeth which took a bit of getting used to and I have a bit of a lisp now.
And finally the kicker for me, in the last two years one of my veneered teeth has died, apparently this is a risk of getting veneers which I was never made aware of at the time. So veneered teeth are more at risk of dying and becoming infected/potential tooth loss apparently. Dentist has said that he thinks the trauma of getting the veneer was what caused that tooth to die on me.
All that said, my colleague's wife has the same teeth veneered due to marks left by her train track braces and she hasn't had any of these issues, so I have had bad luck. I would recommend that you weigh up the risks and benefits and maybe get an opinion from a second dentist and take your time before you make a decision.