Among landscape architects that I've met &/or whose work I've studied, depth & breadth of plant knowledge varies greatly.

Some L.A.s really love plants in all of their diversity and this will be reflected in their work if they are not too constrained by building codes and design standards.

Others have limited knowledge and just stick to a few well-performing favorites. Especially if they are the type who have projects in multiple cities/countries. Then they may need to rely on a local expert to choose plants.

A lot of award-winning designs these days tend to stick to a limited number of plants.