More importantly in RST you can't easily manage several levels of headings. You're limited to 2 or 3, and have to remember which underline character you used for that level.
In rST there are three choices of "line-style": underline, overline, both under and overline. Then there's dozens of possible symbols ("printing nonalphanumeric 7-bit ASCII character"), and the documentation/primer narrows that to a suggested interesting looking 14 options of ASCII symbols. That's a total of 42 suggested options for heading levels, and not even the "limit". There remains the practical limit that Markdown also inherits that HTML only has 6 levels.
There's not a suggested or standard order, so yes there's some mental overhead in deciding which header styles you want to be which level, but a lot of projects tend to have a "lint" guide for that at this point. For instance a lot of projects (and Sphinx recommends) follow the Python Style Guide:
# with overline, for parts
* with overline, for chapters
=, for sections
-, for subsections
^, for subsubsections
", for paragraphs
(You may be confusing Markdown's/CommonMark's own less popular Setext-style headers which only support underline and = for first level and - for second level header.)
You can't make section headers with ##, no?
More importantly in RST you can't easily manage several levels of headings. You're limited to 2 or 3, and have to remember which underline character you used for that level.