I don't think width of leaves has much to do with whether a daylily has vigor or not.
Naturally, if a cultivar has narrow leaves, desert or drought-like conditions (as the op quoted) will affect it sooner than a daylily with more leaf surface, but that is cultural.
I had Highland Lord (Munson-R.W., 1983). It performed poorly here and eventually it died.
I live in an area where at 8AM in the summer, it is regularly 95-98 degrees. A good third of my 750+ cultivars are narrow-leaved diploid spiders, ufs, and round forms. I can honestly say that the majority out-perform most of the wide-leafed tets I grow.