Allston is good when he's writing X-Wing and focusing on writing Wedge, Janson, Hobbie and his own characters. I haven't enjoyed his work in FOTJ or LOTF though alot of that might because I don't like the direction those series have took. Also he sometimes struggles with characterizations of other writers creations, namely Corran Horn as an example. Though to be fair Corran is always at his best when Stackpole's writing him though Zahn has a great grasp of him as well.
It's hard for some authors to work with characters they didn't create or help develop. Look at the Travissty that took place with Mara Jade. (yes, the misspelled pun was intended)
But seriously, Allston still rips Traviss a new one in the writing department. lol
Those book series' themselves were just wrong, imo. After reading up on them, and actually reading a couple novels in the series, they really don't have the "Star Wars Feel" that the previous novels had. It shouldn't be about "What well liked and established character should we kill off now?"