Part of the issue seems to be that there isn’t a clear schedule or plan for constructing this wall. Without a clear baseline, it becomes difficult to evaluate progress against plan.
What seems to happen instead is Fox News and the President pick particular anecdotes that might illustrate his point. For example, there was a particular project to replace and update approximately 20 miles of fencing near the Santa Teresa crossing in New Mexico near El Paso. Once the contract was created, the contractor finished work 47 days before the contractual end date.
Organ Pipe Cactus National Monument is a 517 square mile national monument that includes approximately 33 miles of the US/Mexico border.
Below is the page that describes the border fence. From the 1990s, the park had problems with illegal off-road driving through the desert by smugglers and others. In 2002, a park ranger was shot and from then until 2014 the park was largely closed off to the public. During that time, vehicle barriers were constructed and also a short section of pedestrian fencing. That has made a big difference.
There is not much in the way of roads in the US side, but there is a highway close on the Mexican side. Hence, the recent technique of having an entire bus load of asylum seekers getting dropped off to walk across the border and wait to get processed.
At least 17 people died in 2018, presumably mostly in the desert heat. So that would likely dissuade some of those trying to evade detection, though perhaps less those who want to mostly cross the border and give themselves up.
A somewhat counter-intuitive effect of the wall. These migrants aren’t trying to cross the wall. Instead, because the wall is on US territory, they are standing next to it and waiting for the border patrol to arrest them.
The Rio Grande River forms the boundary between the US and Mexico. This brings into question what happens to that boundary if the course of the river were to change. In 1970 there were a few such disputes outstanding that the US and Mexico settled via treaty.
If the Commission should determine that any of the works constructed by one of the two Contracting states in the channel of the river or within its territory causes such adverse effects on the territory of the other contracting State, the Government of the contracting state that constructed the works shall remove them or modify them and, by agreement of the commission, shall repair or compensate for the damages sustained by the other contracting state.
In practical terms, this seems to preclude building border fence in the flood plain if this may exacerbate flooding in Mexico.
The Lower Rio Grande has a flat wide floodplain and as a result, in some sections any border fence might be placed a mile or more away from the river. Land within this floodplain, whether golf courses, farms or structures are still on US soil and hence gates or gaps in the fence are added. For example this story about those living on other side of the fence.
A large difference between normal flows and flood stage poses a particular dilemma in places where floods are still a recent memory.
Presidio, Nogales, Rio Grande City and Roma are examples cited in the following article where such flooding has occurred. If future flooding occurs in such areas after additional fencing is placed, it may result in international suit.
In Texas, one legislator has filed a bill requiring flooding risks to be studied before wall construction. Though this would be an indirect effect since it would involve the State of Texas suing the Federal government.