Is it ok to use medium abrasive toothpaste on veneers?

Porcelain is a glass like material that is highly resistant to staining and scratching and has properties very similar to enamel. Veneers made in porcelain are the preferred material!
That being said, veneers are not all made the same and can be fabricated with a couple of different materials. Most highly trained cosmetic dentists will only use porcelain when making veneers as they are strong, color stable, and resistant to chipping. There are some doctors who prefer to use composite which isn't as reliable as porcelain. Composites are a resin type material that is similar to plastic. Veneers made using composites can scratch and chip over time. If these are the veneers you have, I would only use a non abrasive tooth paste.

Most glossy- wet looking ceramics have a glaze baked on them which protects them from abrasives.  Some softer ceramics or composite veneers may be more susceptible to micro scratching/dulling but can be polished at your dental cleaning appointments.  It is always better to err on the side of over-cleaning vs under-cleaning and your personal dental office should be able to give you direct feedback on your cleaning strategy and know the science behind your veneers for safe brushing!

To preserve the lustre of the porcelain veneer, abrasive substances can dull the finish.  Ideally, a toothpaste that cleans without being abrasive is best.  Ironically, toothpaste with baking soda is among the most gentle (intuitively would seem MORE abrasive).