Many people say that the author of this anime is too focused on Gon to write the story well, but I disagree. Gon is the main character of the anime, so of course things will be about him. Being the main character, the author has to develop his character more and focus on him more because otherwise, the anime would not be fun to watch.
Characters come and go every arc. Since being a hunter is such a dangerous profession, it makes sense that many of these characters would die. The deaths of these characters is necessary for any of the shows arcs.
The characters Killua and Gon are beautifully written and have their own personality types and special abilities. Killua is not merely a shadow of Gon's but his best friend. They honestly care about each other, both claiming that the other saved them. In fact, Gon literally saved Killua from living the life of an assassin. Killua is not an afterthought; his character is blissfully written and his trauma and backstory connect to his character in a way that makes sense.
Hunters typically jump around from place to place due to the nature of their job. You go where you're needed. It makes sense as to why many of the characters from other arcs aren't heard from again. The same thing applies to the real world.
I have no problem with this anime and I think the world building is fantastic. The plot isn't hard to follow, so anyone can watch this show and enjoy it. The characters develop into real people; not just made up characters on a screen. I would recommend this anime to everyone.