If you have never been down there and spent an extended amount of time there, then you don't know what you are talking about.
The oil boom has had nothing to do with the growth of McAllen. I first went down there with my business, well before the oil frenzy took off. I was absolutely stunned how well developed it was. Good roads, great neighborhoods. Granted it's a border town, but you can tell that big money has been spent to make it a desirable place to live. Trucking and cross border freight is a big industry down there. Oil has added to the enthusiasm but was not by all means the cause. Thus, I suspect there is money down there and it has room to grow. McAllen, IMHO is a real sleeper.
I would invest there in a heartbeat, and it will be interesting how it compares to where I live in Austin. Austin is a tough market for buyers. Virtually all of Texas is hot right now so you just have to be patient and keep looking.