I've heard a lot about this manga and people often made it out to be a difficult read because it is supposed to be very deep. However, this turned out to be a supremely easy read and very enjoyable too.

This does not mean it wasn't deep. Instead, the author struck a fine balance between ensuring the manga remains easy/enjoyable to read, yet keeping the readers thinking about the circumstances and morals of the actions/intentions involved.

However, I think this had a negative effect on the manga's pacing/progression.

I'd say roughly 70% of the manga is fluff wherein readers are introduced to new circumstances to have them think about right/wrong or introduce new complexity to the plot -- complexities, which, honestly, do more to detract from the story than add to it.

Truth is that I enjoyed most of the fluff as I thought they were heart warming, but later on, it really distracts from the plot as the author continuously introduces new characters (and there are a great many of them), new plot points, and etc.

I hate to say it, but the later story just doesn't flow well and makes the story more a convoluted mess than a complex story on morals.

Still, I don't think anyone should take this to mean 'Monster' is a poor manga. As I said repeatedly, it is a very enjoyable read and often does make you think. I'd highly recommend to anyone thinking of reading.