Yes, having to dig a mile long tunnel is totally as easy as waltzing across an open border. And there are ways of detecting tunnels also. When someone doesn't want to solve a problem, there are always a million excuses for why it can't be done.
My point is crossing the border via tunnels is already happening with the current border; I don't see building a wall stopping that.
And yes they still find the tunnels which doesn't stop them being made.
In general while a giant wall might be a deterrent to some, I think evidence exists that it would stop the problem. The debate could be whether it'd significantly drop the numbers, but it wouldn't stop it.
Of course nothing is 100%. But 99% is a hell of a lot better than 9%. There are border walls that work quite well btw, so it's not even a debate.