But Mexico was going to pay for the wall...

8% is honestly impressive. The Mexico / Texas border is 2000 km. If they actually built 160 km of wall that's a real achievement.

But, it shows how unrealistic the project is. 160,000 metres of wall is a massive undertaking. There aren't many other human projects that involve building something that big. And it was only a tiny fraction of the distance they needed to cover. They would have left the hard parts for the end. The hard parts are both hard to construct and hard to legally acquire.

Some sections of the wall would have been hard to construct because of the type of soil/rock, and because of how remote they are from everything. The costs there would probably have easily been 10x the costs to construct in easier places. Good thing Mexico was going to pay for the project.... right?

Then, there's the issue of expropriating private land to build the wall. A lot of the land along the border is privately owned, and even when the owners are die-hard Texan republicans, they're not want to give the government their land so the government can build a huge wall. The stuff that was easy legally would have been the first to get done. The stuff where they needed to claim eminent domain and defend lawsuits in court is another matter. The really tricky stuff would have have been put off as long as possible.

So, it's 8%, but it's not the hard 8%, it's the easiest 8% of what was frankly an impossible project.