Board Thread:Questions and Answers/@comment-36006024-20181107190534/@comment-29325824-20181112164231

No, because Hierro is something completely different from Zaraki's thing. Shinigami don't have Hierro, their reiatsu alone just has to protect from a weaker one but other than that they don't have impenetrable skin. Hierro is an Arrancar-exclusive technique that hardens the skin to immense levels. While Spiritual power does play a part in how strong one's Hierro is, it also depends on how skilled and trained one is. For example, Nnoitra was the Espada most skilled in using Hierro and therefore had the strongest one, despite not even being in the top 4.

That's why Yammy's hierro is weaker than Ulquiorra's. Heck, Yammy was actually weaker than all of them except when it came to reiatsu levels (the only thing that mattered when ranking them) and physical strength. Sonido? Zommari was the best. Firing Cero's? Starrk was the best. And so forth.