What's a leona going to do against a fed rengar? He'll overkill the ADC through locket before she has a chance to cc them. It's a similar case for other assassins. Poppy and thresh are pretty much the only tank supports who can stop certain oneshot assassins from reaching the ADC(wukong and rengar notably).
That's the main thing. Tank supports are great at stopping assassins AFTER they've leaped to the adc and done their burst combo. A oneshot assassin isn't going to just walk into a braum, they're going to dive through him or go invis and kill them under his nose. This only applies to oneshot assassins. When it comes to other assassins like zed, they're about equal at peeling, with tank supports actually being better in a lot of cases.
Tank supports do just as good a job as enchanters at peeling the assassin and even the easiest tank support takes more effort and knowledge to win with than any enchanter bar Yuumi.
That's very, very, very debatable. They take different skill sets and can't really be compared. Anecdotally, i've had a much easier time playing leona than I have on janna and nami, and I have much less experience on leona. If you're duo with a great adc, sure, it is easier to play a soraka than it is a thresh. But if you're not? It is so much harder to have a positive impact on the game as an enchanter than it is a thresh. As an enchanter, you have to do your job perfectly, where as a thresh you can leave your adc to rot and make plays elsewhere. You can abandon your ADC in teamfights and help get theirs killed. If you're playing soraka and don't silence the malphite's flash R when your ADC is out of position they will die and you will most likely lose.
