Is there any good reason for swordstaffs and nagamakis to have both very long blades as well as very long handles? They seems to be the odd-ones out inbetween polearms like halberds/naginatas and swords like arming swords/tachi
To my knowledge, there only exists one contemporary account of sword staves used in Europe, and it comes from a Landsknecht mercenary by the name of Paul Dolnstein.
He claims he witnessed Swedish militiamen using them at the Battle of Elfsborg in 1502, and even provides a sketch.
Now to answer your actual question, yes, these styles of polearm have advantages and disadvantages over both swords and more standard polearms.
Over swords, the advantages are obvious. Greater reach and leverage allow you to strike your opponent from farther away and with greater force. However, as with all things, once you close inside that effective range, the weapon becomes functionally useless.
At these closer ranges, a shorter sword or dagger is more useful.
Depending on the sword of "sword staff" you are working with, it may or may not be shorter than other forms of polearm. Nagamaki (like the one above) for instance have a distinct disadvantage in reach over things like Yari or Halberds, however a European sword staff like the one outlined by Dolnstein may not.
The true advantages and disadvantages stem from the size of the blade.
(Philadelphia Museum of Art)
(Metropolitan Museum of Art)
and many other forms of polearm tend to have short, stout blades. The weight of these is more concentrated, and therefore imparts more force to the target upon impact, however there is less edge to strike with.
One of the easiest ways to deal with a polearm when using a shorter weapon is to wait for the polearm wielder to swing, and move in under the weapon, blocking the polearm by it's haft and directing it away from you.
This is much harder with a weapon with a long blade, because of encountering wood, you're likely to just encounter more blade, putting you in more danger of injury.
I hope this has been helpful and interesting, cheers!