I use a lot of these for growing out fry... as well as hospitalizing fish that get beaten.
They have two achilles heels:
1) Bigger fish can eat / try to eat little ones through the bottom
2) The little plug things that hold the aluminum straps to the tank can pop off...depositing the contents into the main tank.
Because I'm usually draining 4-5 tanks at a time, I sometimes lose track and drain tanks below the nets...
An approach that works for me is to set up a bare bottom tank (in my case a couple of 2'x2'x1' - 33g) with a large sponge or 3 and a box filter...along with 4-5 net breeders for various batches of fry. Not having small fry or gravel in the tank itself makes it easy to clean. Not having big fish in the tank keeps the fry safe. And you can have several seeded sponges for new tanks. Keeping the fry in a net also allows you to maximize the amount of food that fry find and eat (and minimize the amount of food that they miss and pollutes the water).
Matt