I'm not familiar with most of the other "things" like #Pleroma, #Akkoma, #MissKey, or #CalcKey, but I'll guess that those "things" (?server application projects?) are using a different sub-set of #ActivityPub protocol to service a different set of use case foci?
Different use cases might have different focus, such as microblogging (#Mastodon) or bulletin boarding (#kbin subbing for #reddit) or video streaming (#PeerTube subbing for #Youtube) or other use subset of social function?
In analogy to your last paragraph, if something doesn't work well on the Safari implementation of #HTTPS, it's Safari's problem.
Thank you for your thorough answer. While I personally don't know about a lot of the examples you provide, it made me think differently about how to frame questions.