Zaheer was not a radical. Admiral Zhao was.
At its foundation, Zaheer's worldview echoes Heraclitus--the universe exists in a constant state of flux, and conflict between opposites is the engine of reality. Human authority is therefore not merely unjust; it is metaphysically illegitimate, because it seeks to deny the bloody womb from which it emerged--conflict. If this is where Zaheer's philosophy ended, he would be remarkably coherent in his opposition to the Avatar as an unaccountable cosmic authority figure.
Yet despite knowing of Unalaq's plans and the cosmology surrounding Vaatu, Zaheer never treats the prospect of spirits reclaiming the mortal world as a form of domination. He instead interprets it as a desirable restoration of nature's proper balance. There is a hidden premise smuggled in from the white lotus here--the notion that the spirit world possesses intrinsic legitimacy unavailable to the human civilization. So without realizing it, Zaheer is working to construct the exact sort cosmic hierarchy envisioned by Korra and the White Lotus. A society configured around the diktats of mercurial, inscrutable spirits. His disagreement with Korra ultimately boils down to which spiritual order should govern, not whether spirits constitute a legitimacy source of authority.
So philosophically, Admiral Zhao is more radical than Zaheer. Zaheer merely transfers authority from despots to spirits. Zhao denies that spirit possess any sovereignty at all and treats them like strategic assets. So while the Fire Nation and Zhao might have failed politically, their historical mission in spreading the industrial revolution, scientific inquiry and human mastery over nature succeeded. For a brief, shining moment, Admiral Zhao was the World Spirit astride an Ironclad.