Some of the issue is that WordPress has a huge number of plugins which adds a lot of additional complexity (especially for Micropub clients) which isn't always going to be handled by every client.
As for webmentions, they're being bolted onto WordPress which doesn't have (or allow) a custom comment type, so they're jury-rigged in the best manner possible. There is still ongoing work, especially with the Semantic Linkbacks Plugin which does a lot of the heavy lifting here, if you've got thoughts/ideas, you should certainly weigh in on those Github issues as they are evolving.
I know there were a handful of quirks that have been changed in Known in the past months to fix some issues with microformats that were being parsed incorrectly, and thus caused other platforms like WordPress not to let them display as nicely when received. I think most have been merged and a new release of Known was pushed yesterday and should hopefully clear up some of these issues. I think the developers of the WordPress plugins and even the Known community are very responsive, so feel free to jump in with feedback, suggestions, or even pull requests on any of the pieces which are all on GitHub.
I'm sure there will be some remaining rough edges, but in general a lot of them have been smoothed over in the past year and most improvements now seem to be geared towards making the suite of tools more user friendly or to better extend the functionality.