when not to use net.ipv4.tcp_{sack,fack,dsack}

Does someone know of a good explanation / link for when

net.ipv4.tcp_sack, net.ipv4.tcp_fack, and net.ipv4.tcp_dsack

should not be used? All the doc I've seen says they're "almost always
a good idea" but no description at all about when they're a bad idea.

I'd like to get past all the handwaving.

.