Oh boy...
Anti cav are units intended to trade cost effectively with mounted, but pretty much nothing else. Problem: the number of units on a civ6 battlefield negates the “cost effective” part because it’s somewhat uncommon that you will actually be able to turn that into greater numbers. Also they aren’t actually that cheap in most cases. Pikes at 180 vs knights at 220: you need 5 pikes to get an equal production cost with 4 knights. And you’re only at +3. But then anyone who can make knights can build swords for 90, which beat pikes by 5.
AT crew at 400 vs Tanks at 480: it’s 6AT crew to 5 Tanks. And they trade at even combat strength!!
Then, they have a dedicated counter that is exceeding efficient at demolishing them. So they deserve their reputation. If melee didn’t rip off their heads and dump chamber pots down their necks, they might be able to serve as a generic mass of meat countered by ranged fire.