Not bad for a middle-grade graphic novel, but, for some reason, I was expecting something more mature. I don't know why, maybe I was thrown off because it wasn't in the kids section of the bookstore, and instead in a separate graphic novel section right next to the adult fantasy.
It seemed too fast-paced for me, which might be a side-effect of it being a middle-grade book; 10-14 year-olds don't have the best attention span, so it should be fast paced, but as an "adult", I think it got in the way of a good story. There were some moments that should've lasted longer to give them the proper weight, and it was so fast there was no room for foreshadowing. For example,

the kids meet Four Dollars, a guy with a boat and criminal ties, on page 145, and get scammed by him twice in the remaining 17 pages of the chapter. That's not too bad, right? Well, they hand Four Dollars a map of where they need to go on page 170, and he realizes he knows who they're looking for. He decides to scam them again by page 173, and the kids begrudgingly agree 2 pages later just in time for Four Dollars to look longingly out the window on page 177 (this is the only foreshadowing for what is revealed later, but this trope is so played out that you probably already know what the twist is). Page 179, Four Dollars leaves to look for the person the kids are paying him to look for, and comes back for the next 3 days (and 3 pages) increasingly drunk. On the 3rd night when he falls unconscious onto the floor, the kids find a note in his pocket that reveals that gasp Four Dollars is the uncle they hired him to look for (page 185)! But they don't discuss this huge revelation much before going to sleep. Uncle Four Dollars wakes up the next day and they all have an argument about stuff, Four Dollars goes out again on page 188, and the kids immediately follow him on 189. The kids get attacked by zombies or something on 192, Four Dollars saves them on the 193, and all is forgiven on 197. 
I know it's a bit harsh to criticize it like that, listing the page numbers and all, but I do think it would've been better to have more than just 48 pages to sow distrust in Four Dollars, and only 12 pages to sit in their anger at his lie before it all goes away! And, remember, these are 6-9 panels per page, and 1-2 bubbles per panel, so it takes a lot of  pages to get stuff done, but you go through the pages really quick. 
Maybe, instead of having TK randomly pick up the kids in the beginning and hand them off to Four Dollars in the middle, it should've been Four Dollars from the start. Maybe he wanted to find and make up with Odessa after he left the Gogu and was too shocked by running into the kids to say anything (or, if TK needs to be there because she's gonna pay off in the sequel, have Four Dollars travelling with her as his guide). That way we have more time to get to know him, properly foreshadow the big mid-book twist that he's their uncle, and make it hit harder when he dies at the end.

With that said, I think I did like this book just enough to keep my eye open for the sequel. It's got some good worldbuilding, an interesting story, and fun, diverse characters to take you through it.