Yamato's mokuton abilities are weak because he was part of the very first wave of experiments. Oro was basically an amateur when he was experimenting on those 60 children (I can't believe they let this guy live)
The next step was what he did with danzo some years later, but that still wasn't perfect, it was perfected with Madara's Edo Tensei tho. Kabuto says what he did with Madara was on a different level than what Danzo "prototyped" so all those years later and Danzo was still at the "prototype" stage. That means Yamato is even more basic than what Danzo had
If Oro was creating first hokage clones now they would be way stronger because he's perfected how to do it.
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The next step was what he did with danzo some years later, but that still wasn't perfect, it was perfected with Madara's Edo Tensei tho. Kabuto says what he did with Madara was on a different level than what Danzo "prototyped" so all those years later and Danzo was still at the "prototype" stage. That means Yamato is even more basic than what Danzo had
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If Oro was creating first hokage clones now they would be way stronger because he's perfected how to do it.