It took me a while to rate this book. I don't have any license to talk about what makes a middle grade book good except that I read a lot of books at that age. It's hard for me to critique a book meant for non-adults as an adult, and I would love to ask my kid self what I thought of it back then.
I first read this back in elementary school, 9 years old I think. It was for an advanced student class and we read a number of books for it. I remember my teacher being very excited about this one but I honestly don't remember my reaction to it. That's why I wanted to reread it now, as an adult. 

I definitely understand why this book won an award and why it's highly regarded for children's historical fiction. But to be completely honest, it seems a little overrated. It's important for kids to understand topics like immigration, classism, racism-- and to hear stories from many different perspectives. Esperanza's perspective is not a very common one in children's literature (especially when it was first published), so it stands out. But the representation alone doesn't make a story. I think this one is very good, but not great. 

I like the beginning of the story and found it very easy to get into.

We jump right in with her father's death, and add villainous uncles for a reason to flee. (The uncles never try to find them or make any appearance again, which seems a little too easy to me but whatever)

The description of getting work on farms focuses much more on Esperanza, and her slight struggle to learn simple tasks since she comes from a rich background, than on the treatment of workers who are seen as disposable by their employers, despite their hard work and loyalty. It does a good job of touching on the strike and immigration coming to cart people away, which I suppose is all you can really do in a children's book. But there is no nuance or explanation which makes me think this book would be much better for kids reading it in a class situation, and not all that educational for kids reading it on their own. This is a book to make a foundation for classroom discussion, but it doesn't do much speaking on its own.  
Esperanza as a character isn't all that interesting or compelling (at least to me). She's just a kid, and really does handle her situation very maturely and grow throughout the book. I understand middle grade books can't be all that long, but kids can handle much more complex emotions than adults often expect them to. This book could definitely have fleshed out her character more with detail in her thoughts, emotions, and reactions. 

All this to say it's a well written book from an underrepresented perspective, but not all that memorable or impactful. I'd probably recommend it for classroom reading and it certainly wasn't a bad read for an adult. But it's not one of the greats either.