This is not strictly true. The following source markdown is rendered as in the screenshots below:
> [!NOTE]
> Admonitions work with no indenation
* test
> [!NOTE]
> but break at two spaces
* test test
> [!NOTE]
> but four spaces of indentation are not always rendered as code blocks.
on GitHub
via Pandoc
$ pandoc -f commonmark -o output.html <input-file>
$ cat output.html
<blockquote>
<p>[!NOTE] Admonitions work with no indenation</p>
</blockquote>
<ul>
<li><p>test</p>
<blockquote>
<p>[!NOTE] but break at two spaces</p>
</blockquote>
<ul>
<li><p>test test</p>
<blockquote>
<p>[!NOTE] but four spaces of indentation are not always rendered as
code blocks.</p>
</blockquote></li>
</ul></li>
</ul>
I assume you mean that setting the left-margin on the blockquote to be exactly equal to the left-margin of list items is difficult? (presumably, the width of two spaces in a monospace font.) or is there something else to it?