Haven't played a Shadow to a high level, but this goes against everything I've ever heard from a Shadow player.
they are the easiest tank to play. It said nothing about playing well or badly.
Threat is no issue with shadow/assassin tanks, meaning it's one less thing to worry about. Where as for guardian/juggernaut, it is their biggest issue to date. If you dont have a threat issue with Guardian/juggernaut, you are ready to tank everything in game, assuming your stats are in order.
playing a shadow tank well, requires skill. you need to know which boss abilities does this and that type of damage and then what cooldown to use. this may require testing/training. But as you train this, your healer or dps shouldn't be afraid of nuking, cause threat shouldn't be an issue, unless you dont guard, for example a marauder in full Campaign/dread guard if you wear tionese/columni.