I've heard that it puts a super-heavy coating on your hair that can take a good long while to wash off. The old Silkience shampoo used to do something similar. Beauticians would always ask you if you used it and refuse to color or perm the hair of anyone who had used Silkience in the past two months because of the coating it left behind.
About 10-15 years ago there was this fad for "barnyard cosmetics," i.e. stuff used on farm animals like Bag Balm. I think the Mane N Tail you find in Walgreen's is very different from the stuff that's actually sold for horses. The stuff for horses has a blackening agent that would make your hair tarry. But that's just Internet hearsay.