[stass]

What's wrong with Mimosas? They smell so nice in spring!

[astura]

They are invasive (in the US) and grow like weeds completely taking over and crowding out native species. Having many of them screws up the nitrogen level of the soil further crowding out/killing native plants.

This alters the ecosystem.

[sarchertech]

In addition to what everyone else said, they are nitrogen fixing, so their leaf litter creates too much nitrogen for most native plants to grow.

[hammock]

They are invasive, take hold and hard to eradicate/control, the flowers make a mess, and they are easily damaged in storms

[pvaldes]

Their invasive dark side is directly triggered by fire.

[hollywood_court]

Other commenters have covered most of the reasons why they are awful, but my main complaint is that they will completely take over your garden beds and wooded areas if left undisturbed.

[deciplex]

Like anything else they're fine - the problem is when you drink too many of them.

[lotsofpulp]

In PNW, they look dead in the spring. The leaves don’t even appear until late May, and flowers bloom in late July at earliest.