If you just go and buy a card and use the normal firmware you're gonna get banned. Cheat creators make custom firmware to avoid that. It might be that Faceit is small enough to investigate cheaters thoroughly to get most of them, and with their reputation it might discourage most to even try. But I don't think that scales enough for big games unless you have Riot money.

DMA cheats are not detected. What happened is thousands of cheaters all bought firmware from the same guy, and Riot was able to determine via stats that this group of people with the same obscure "network card" had outlier stats, and they banned them all. DMA is by definition not detectable, but human idiocy is.