

Having Sam lose his best friend might have been a bit much, in particular.

In a kids' book, I suppose you don't want to linger on that part terribly long, but that may be part of the problem, as well. This all happens in a very short timeframe, of course, yet Sam's caretaker is complaining about him spending all his time with a dog the very day after Star boards (she's also not the best caretaker, but guardians in books rarely are, or else shenanigans could not ensue), and later, the ending is rushed and, more importantly, the night of the sinking is all extremely abrupt.

I'm in agreement with my 2016 self here, but to add a little more explanation on a second reading: part of the problem with the emotional depth is the pacing.
