As mounted infantry without serious aerial opposition, maybe. I include drones in that category.
Armor (well, the tier below the M-1 level of AFVs,) can be defeated by man portable weapons but the danger of Arty guided by Drones or aerial observers is extremely threatening too.
Third world scenarios come to mind (or apocalypse - but that enters SF/Fantasy realms.)
Horses are incredibly costly to maintain in the field, fragile in a modern environment, and the troops require training not common to many nations/cultures.
Gracias,
Glenn