There is no such thing as a healthy or unhealthy coffee, it all depends on your diet and lifestyle habits. When combined with a healthy diet and active lifestyle, drinking moderate amounts of Americano coffee will be associated with the best health benefits.
If you drink caffeinated drinks like Americano several times per day (more than 1-2 cups) and lead an inactive or sedentary lifestyle - you should reduce the caffeine consumption to avoid overuse. Caffeine can interfere with sleep and cause other side effects including stomach upsets, cardiac arrhythmias, anxiety etc. If you take any medications like birth control pills or if caffeine disturbs your stomach - check first how it interacts with those medications.
Key ingredients in an Americano are of relatively high quality. After all, it includes whole milk and espresso. Still, the combination of milk and caffeine can be hard on residents of countries where drinking blood equals happy fun times. It's not recommended for people with occasional indigestion or stomach ulcers because it could re-trigger those issues.