There's nothing wrong with the concept. The problem is their availability. The recasts themselves look fine, until you remember that you have two tanks. Your team has an invuln available every 2-3.5 minutes. When they're so available that you can mitigate every tankbuster with invulns alone, then there's a problem. When they're so available that you can use them to cheese other mechanics, then there's a problem.

What makes conventional cooldowns different is that you have to actually know how much damage the tankbuster does. Can I get away with just Rampart, or do I need a 30% cooldown? Do I use one big cooldown, or stack two? If every tankbuster does the same amount of damage, then you don't really have to think about any of this. But if the damage on each progressive tankbuster ramps up, suddenly we have to ask ourselves how best to ration out our cooldowns. That's usually where invulns start to have meaning.