first of all many of them just don't work very well; often they will port over one half of a conversation or try to bring over a feature like QTs that just don't exist here.
secondly social norms are just different on fedi; stuff that comes from twitter won't have CWs or good image descriptions or sensitive images marked as such.
third is that they give off Bot Vibes; you can tell an account isn't really a person, the person is somewhere else. usually this makes it feel like they're not actually interested in having a conversation, which is what the rest of us are here for.
overall it's just awkward and jarring.
(note that crossposting in the other direction is fine and encouraged)
@technomancy ok :) Thnks for answering in such detail!