If you make them right you should be fine. What a lot of people do is make basic Victorian (front-groove) nocks, and simply turn them sideways, and use them with a normal sized string loop and they look bizarre and can also fail. The string groove needs to be shaped quite carefully (and is very different to the Victorian style) and the string loop needs be really tiny. If both of those elements are ok, then it doesn't matter what horn you use.
With large loops that hang down from the nock, buffalo horn has a tendency to split, whereas cow horn doesn't.
Here's a quick rough sketch of how the groove should be made (if you use a knife it happens almost automatically, whereas if you use a tile saw or something you have to really work at getting that shape right)
And here's a nock made in buffalo by a friend of mine entirely by knife, which is (almost) identical to the MR nock and should show you the sort of thing I'm waffling on about.
I'm probably making it sound more complicated than it is. Give it a try on some scrap horn and see how it turns out.
.....or use front nocks like the Victorians did...