Good Against: Nothing.
Bad Against: Everything!
Why use them? Use them to carry fighters, always send these things away before you have a battle otherwise they will take damage and most of them will die. These things are not as vulnerable as Carriers and carry more fighters per credit than Carriers, but they are slower.
Advanced tactics Fleet carriers are better than carriers when dropping fighters on enemy astros. Otherwise, everything said above in the Carrier section applies here as well.