[quote name='The Holy Pretzle']For competitionists who want this for Achievement's sake; there are a few achievements that are impossible to get. Some achievements require killing some animals or people with a certain DLC weapon in the story mode; which you can't do if you buy this disc or own RDR vanilla.[/QUOTE]
On an interesting flipside, for non-competitionists (???), there are quite a bit of RDR achievements that you can get by only having the Undead Nightmare disc. I've gotten 9 so far, but only 3 of them are actually from the Undead Nightmare content of the game.