The API is still not completely fixed, which is why all sites that automatically parse data from characters aren't working 100% reliably. Especially in terms of survivability, toughness and EHP aren't good measurements (it's interesting though, because for 10-15 years EHP was widely regarded as the main survivability measurement). What's better is to calculate your mitigation based on the in-game values you see on your character sheet (because those include paragon and so on). How much is enough? I say there's never enough, I always want to get more mitigation, but of course there are trade-offs. Try to keep your life low (in most cases 300k are enough, I even know WoW wizards who play T6 with 250k life) and focus on mitigation as much as possible.
There are many mitigation aspects that are often overlooked: set bonuses from Aughild's or Blackthorne's; armor on slots where it can roll high (helmet, chest, pants; not worth it on amulet, though); elite damage reduction on chest; and secondaries (melee/ranged damage reduction on bracers and chest, crowd control reduction on jewelry and helmet).
While I agree with you on that point, I believe that toughness + hp can provide a quick and reasonably efficient comparison, which is especially useful for players who haven't had an in-depth look at the system already.
Obviously, it won't include elite/melee/ranged reduction or specific affixes like 4p BT/Ice Climbers, but it's a step in the right direction, and it makes the difference between 10M toughness with 500k hp and 10M toughness with 250k hp pretty clear