The modern Unitarian branch of Christendom began within a few decades of the Protestant Reformation, so it wouldn't make much sense to consider it a whole separate reformation event. What value do you see in such a suggestion?

Did you mean few decades before or few decades after? I don't have an opinion about this, I want to know if experts on the subject think it is a correct distinction and why, so according to your phrasing I don't see a value besides that it might be an elegant approach to arrange data or to describe history.

Some quick research with Wikipedia indicates that the 16th century Unitarianism was a schism from Protestantism; if it warrants being considered a separate reformation then it's not clear that Lutheranism and Calvinism should be placed under the same umbrella as each other.

I would classify Unitarianism as a branch of the Radical Reformation, which could be classified as a second reformation, which tried to reform the magisterial reformation (Luther, Calvin, and Zwingli). The Radical Reformation was made of Anabaptists, who believed that infant baptism was useless and that they needed to rebaptize themselves. Modern-day Mennonites and Amish are branches of this tradition, as are the Unitarians of the 16th century.