I wouldn't say Markdown is simpler. It has fewer features, but the core experience is about the same as reStructuredText.
RST header markup is more readable in plain text. In Markdown, the less important a header is, the more hash marks. In RST, I am free to make less important headers have less visual weight.
Markdown also supports non-hash mark headers, similar to RST, but it is a bit tougher to deal with its h1 -> h6 progression than RST. The RST rules are clearer and standardized, whereas Markdown's are not and especially vary between engine implementations. One more area where CommonMark standardization may help some time in the future, but doesn't quite impact today.
They're standardised but I wouldn't say they're clearer as the "level" of a specific over/under symbol only depends where it first appeared in the document, I've seen projects where two documents side by side used different underline symbols for the same depths, that gets confusing.
I'd like rST/Sphinx to be stricter on this point, maybe I should open an issue/PR allowing upfront definition of the over/underline hierarchy for the project.
Agreed, RST headers are too "flexible". They're also IMO quite annoying to write and maintain (exactly matching the title length) or ugly (overlength).
Plus, solving inconsistencies with simple head math: $ to the end of lines to get the character counts from the two line and say, add 3 with 3i=[Esc] or subtract 5 with 5x. With over and under decoration you can . the other side to fixup both.
Header significance to visual weight inversion is the same deal in the lauded orgmode. In both cases, the trick is to see the hash mark weight as indentation instead of significance, or, preferably, configure your editor to weight the headers using font customization.
RST header markup is more readable in plain text. In Markdown, the less important a header is, the more hash marks. In RST, I am free to make less important headers have less visual weight.