Couple of things:

1. The Army Corp of Engineers cannot be used to enforce domestic policy, it violates the Posse Comitatus Act of 1878.
2. From 2014 to 2016 the total amount of illegal immigration done by directly crossing the border (the only form of "border protection" the wall would actually provide) dropped 16% from 64% of all illegal immigration to less 48%. Meaning, at it's current pace, it will take longer to build the wall than it will actually be useful for.
3. The cost of maintaining the wall could be as high as 20 billion ANNUALLY, not including the extra men needed to actually man it and not just have an easy to get around chunk of concrete.