My biggest worry about hp is that it might be impossible to win against a defense with a lot of hp and good blocks on flag and towers. That's why I think it could be a good idea to add more rounds to battles if the defenders' total hp goes over a certain limit, let's say over 400k on medium for example. Maybe then the attackers could do something other than try to take the flag.
Besides that, I don't think there's any big issue with hp tanks. As long as they're somewhat balanced between the main sides there's gonna be good battles. And that usually comes down to the 2 biggest alliances on a world. People gotta work to keep things balanced and interesting.