Lattes are not unhealthy if consumed moderately. They can contain positive elements such as good fats, they are high in antioxidants, and the deliciousness of the latte may help with mood elevation.
The only reason it is unhealthy to ingest coffee on a regular basis is because of its caffeine content. Coffee's caffeine has negative effects like sleep deprivation, stress, addiction risk (a mental health issue), raised blood pressure (aka hypertension), and erratic heartbeats (arrhythmias). Some people will become hooked on it; some enjoy one cup per day; others might drink 3 cups per day; others might feel like drinking more than 5 cups per day for many consecutive days without rest periods in between drinks.