Mesh and knit uppers are fantastic if you ride in hot weather or sweat heavily. They offer maximum airflow and a sock-like, wrap-around feel that many riders find more comfortable than traditional materials. For summer road riding or warm-weather mountain biking, they're hard to beat.
The trade-off is protection and durability. Mesh uppers won't shield your feet from rain, cold, or trail debris like synthetic leather or GORE-TEX would. They also tend to break down faster with heavy use, especially in mountain biking where abrasion is common.
Choose mesh or knit if your priority is hot-weather comfort and you don't ride in wet conditions. Skip them if you need an all-season shoe or ride in muddy, rocky terrain where foot protection matters more than ventilation.