Mastodon came after Hubzilla. Hubzilla was built on its own protocol and Hubzilla added the ActivityPub protocol long time back to connect to the whole Fediverse (not just Mastodon).
You know that, I know that, but there are not exactly few people on Mastodon who don't know that.
Even when they learn that the Fediverse is not only Mastodon, much to their surprise, they tend to believe that if Mastodon isn't the only Fediverse project, it must still have been the first. And everything that isn't Mastodon is either bolted onto Mastodon or an intruder.
Don't see what issues Mastodon users may have with it, as it has connected for ages.
Disturbingly long posts. Non-Mastodon users are banned for exceeding 500 characters once. And I'm pretty sure that Mastodon users try to have non-Mastodon users sanctioned by their admins for breaking this unwritten Fediverse rule.
Quote-posts. For Hubzilla users, it's called sharing, and it has been the equivalent to Mastodon's boosts since 2015 because Hubzilla can't boost. Not until Hubzilla 9 is out. For minorities on Mastodon, it's the exact same thing as the quote-tweets that were used on Twitter to harass them. And Hubzilla doesn't let them opt in, not even opt out. Hubzilla can quote-post anyone on Mastodon, always.
I'm not even sure if every last first-wave and second-wave Twitter refugee who got used to Mastodon 3 is used to seeing things on Mastodon that were absolutely impossible on Mastodon 3 such as text formatting.