I thought Zahn did just fine with the character growth, especially with Han. He introduced him struggling to take on his new role as a diplomat and husband, trying to reconcile his now "adult" responsibilities with the devil-may-care smuggler persona of the past. Leia was written a bit weaker but she still has grown a lot as a character, where we see her actually acting as a politician while trying to learn about the Force and her new responsibilities as a mother. These are very different characters than who we left off with in the OT--yet it's perfectly believable that in 5 years after ROTJ, those characters would grow up to become these people. I wouldn't say there is as much of an "arc" like in the OT, but it's pretty unfair to say that Zahn didn't handle character growth and character change--he did it better than any other EU writer I have come across.