A sleeveless jersey can be great for maximum cooling in extreme heat, but it comes with trade-offs you should know about. You'll get unmatched ventilation and shoulder mobility, which is why sleeveless jerseys are popular for triathlon and indoor training where airflow is the top priority.
However, sleeveless jerseys leave your shoulders completely exposed to the sun—something to seriously consider on long outdoor rides. They're also less aerodynamic than fitted short sleeves and tend to look out of place in road cycling culture. For mountain biking or casual riding, that matters less.
For most hot-weather road riding, a short-sleeve jersey in a lightweight, mesh-paneled fabric paired with sunscreen gives you better sun protection and a more versatile piece overall. Save the sleeveless option for triathlon, indoor trainer sessions, or those brutally hot days when cooling trumps everything else.