I don't think there's long term evidence for negative problems. However, high dosages of caffeine are dangerous and have killed people. Those dosages are easily achieved through the likes of energy drinks.
High dosage of water have also killed people. And easily achieved through energy drinks? I strongly doubt that. Most energy drinks are not any stronger than coffee
Chronic long-term use of high amounts of caffeine (>600 mg, or two Starbucks Grande coffees) are linked with: bone density issues, sleep issues, anxiety issues, cardiovascular strain, stomach acid issues. They aren't super bad, but definitely not harmless.