When would you recommend lumineers instead of veneers?

Normally the ultimate goal is a beautiful smile. If Lumineers are being offered, ask to see some finished cases done by the office in question. Rarely are there Lumineers cases that are beautiful or fantastic, and you will see after seeing the pictures. Even some of the official marketing photos from the company are just so-so.
If tooth conservation is the issue, most veneer cases can be very conservative but would require some tooth reduction. This is replaced with porcelain, so the end result is NOT a weaker tooth, just an altered one.

I've done and placed both traditional porcelain veneers and the most recent generation of Lumineers (IPS eMax).  If you’re just looking to have WHITE teeth with minimal tooth details, then Lumineers is the one. All my of Lumineers patients are happy with their results.  They're stronger materials (Lithium Disilicate) than porcelain veneers and patient teeth doesn’t get ground down as much.  A huge perk is that current Lumineers cases are mostly "no prep" technique with patient's teeth with some cases "minimal prep" (some recontouring of teeth)!  Not all patients are candidate for Lumineers so make sure you visit an official certified Lumineer doctor for an complete evaluation.  Locate a certified Lumineer doctor in your area. If you want LIFE-LIKE teeth with different shades of white and other colors in the same tooth, then feldspathic porcelain veneers are for you. Because porcelain is more fragile, they need to be thicker than Lumineers. Because of their thickness, more of your tooth structure will need to be removed/ground down for the porcelain veneers to not look bulky once they’re on your teeth. Porcelain veneers are more detailed than Lumineers veneers. So it all basically comes down to do you want durability with minimal teeth grinding or do you want the most realistic detailed looking teeth and don’t care about your teeth being grind away?