One of the things that has stopped me from getting a barrel is the fact that I assume at some point it will go sour. I do not like sours, so it would be useless to me at that point.
The possibility is always there and the probability of a barrel turning sour (whether in a pleasant or unpleasant way) increases with each use. Brewing your own barrel aged beer is almost certainly cheaper than buying the equivalent amount of beer if you are able to obtain 2-3 turns out of the barrel. That is true even at the increasingly inflated prices of 5-15 gallon spirit barrels. The probability that your barrel turns sour that quickly is unlikely if you follow decent barrel care procedures. With good care you could probably use the barrel for years before it needs to be decommissioned.