I think Webmention is viable at the moment just because of the low adoption across the web. Just like with pingbacks and trackbacks, any kind of manual moderation becomes infeasible when valid posts get swarmed under thousands of automated spam posts.

The typical verification step (check if the originating site contains a valid link) is trivial to work around. One attempt to address that is Vouch [1], but I believe it's largely untested in the wild.

[1] https://indieweb.org/Vouch