Why does the Haakaa get such mixed reviews?

Because reviewers use it for different jobs. Threads on r/breastfeeding and r/HumansPumpingMilk note that as a passive let-down catcher during nursing it works well for many parents, but as a primary pump it underperforms, and aggressive suction can overstimulate supply or cause oversupply problems. The disagreement in reviews is usually about expectations, not the product itself.