The most powerful warrior.

I disagree.
But I won't throw stones; I'm not a gremlin who throws stones....

If we're talking about usefulness, what do you mean by that?...

And it has been mentioned several times here that there is a big difference between a regular Phoenix and a summoned Phoenix. A regular Phoenix will always have stable characteristics and can be on par with a Creature of Darkness. But a summoned Phoenix, its strength grows with the hero's level (depending on his magic skill), and it can claim the title of the strongest unit.
Because its life and damage depend on the hero's magic skill.
In the case of single units, this is very important, as well as the 20% damage reflection from the shield.