Of course, name-brand fuses are tested to very specific tests, and you get to see the plots of how the fuses are supposed to melt for Bussmann fuses. Then he takes out a generic assortment of fuses he bought at a hamfest. No Bussmann fuses in that batch!
Comparing the generic fuses with some from Bussmann and Littlefuse, they all work fine to carry current. That isn’t the problem. The problem is when you feed the fuses 20 A and expect them to clear. A 5 A generic fuse carried over 20 A for a very long time, and, as you might expect, it got very hot. We kept waiting for the fuse to blow, but after three minutes, he gave up.