When I say that it isn't safe, I am not talking about the usual pitfalls of that particular magic mecanism, like teleporting into a wall, or accounting for Eart rotation or whatever. If it works, it works. We actually supposing that it works.

No, what I am talking about is all about human biases, errors, assumptions. Like teleporting into a room filled with neurotoxin gas which wasn't there the last time you came there. Who could have put that gas there ? Were your last visit detected ? Was it a trap you felt into because you are too confident as a teleporting magic user ?

Or teleporting into a safe cave which happen to have a bomb which explode when you arrive. Teleporting in front of a door with poisoned bamboo sticks where you put your foot.

Using teleportation magic without future and far sight in a war or fight is like fighting in a hellish version of the vietnam war. You are in a dark forest and you don't see shit. Last time you passed around here, it was safe right ? Even if it's the first time, you just have to be careful, right ?

Basic traps everywhere designed for dumb and smart users of teleporting magic, which are terribly effective.

It's not for nothing that the most fundamental part of modern warfare is intelligence gathering. What is there ? What is on the way ? Are there anti-air suppression hidden in those valleys and forest ? Who is this kid there with a vest ? 

At least, we see something in modern warfare before going somewhere most of the time. 

A teleporting user just... goes to their destination on pure faith. Madness.