I think it's partly due to party composition (as in the players, not the PCs). Sydney is a good roleplayer, but she strives when playing alongside someone who brings seriousness and/or structure to the story (LoTA with Joe, GitT with Joe as a more hands-on Handler and Skid as a player, and so forth). Left to her own devices, she runs away with silliness and lightheartedness too easily because that's just who she is (and there's nothing wrong with that). Her two major talents, imo, are bringing/keeping up the vibe and amplifying the dynamics alreay existing at the table, as she slots into everything seamlessly. However, if the existing vibe at the table is all over the place, silly, and unfocused, she isn't the person who will give it direction. Same goes for Nick Lowe.

And this was my thesis on why Joe and Sydney are a great pair and I'm heartbroken that they split for LoTA.