I think it's not to say that no relational databases has any of these features.
But I think it's more exceptional than the rule.
Relational databases are primarily designed for relations. And saving your objects as documents simply is a different model.
That doesn't mean one is better than the other, it's two alternative ways of achieving things.
Indexes can be created on fields within JSON documents for faster access.
Online ALTER TABLE doesn't take any long running lock, so it can run in the background without affecting your queries.