hello; They should be good in that they do not rust nor absorb water. My main concern would be the possibility of the concentrated weight deforming the plastic.
I have used metal stands many times and have tried some plastic types. The legs on my stands are often simple angle iron with no base plate, at least for the smaller tanks. Tanks up to 55 gallons. I have found that the edge of the leg to have sunk into the shim over time.
A 180 gallon might weigh over 2000 pounds with all the stuff. That is around 500 pounds possible on each leg.
A way to use plastic might be to have a metal plate of some sort on top of it to spread the weight from being concentrated at a small point.