In case the reasoning for my above post isn't clear to anyone:
The US-Mexico border is approximately 1,900 miles. 1,254 of those miles are the border between Texas and Mexico. Go pull this border up on Google Maps and check it out closely. The reason the border is so crazy has to do with the fact that it's based on the Rio Grande. You know where it really sucks to try to build a 30 Foot Tall (or higher) wall? In a river bed. So where is the wall going to be built along 2/3rds of the border? In Mexico? HAHAHAHAHAHA good luck with that one
In the US? Well that creates quite a few problems. Unfortunately for the wall plan, the area along the Rio Grande has been heavily built in several areas with both commercial and residential interests. But what about eminent domain? Sure that's one way you could do it, but I doubt Texans are going to look to kindly to a gigantic Federal Land Grab based on Eminent Domain. Along with the fact that this would likely be a rather lengthy process. Then there is the matter of the portion of the border that runs through Big Bend Ranch State Park and Big Bend Ranch National Park. This likely won't raise as big of an issue as private land, but still will be another hurdle to clear in the planning of the master wall.
So what we'll most likely be left with, if a plan to build a wall is ever even started, is an incomplete eyesore in random spots along the US-Mexico border.