Everyone mentions how visible mines are easy to disarm, but disarming (especially with a basic engineer) takes time and the repair is easily visible. It's very likely that the engineer would be discovered before they can get the mines disarmed.

One nice placement that has worked well for me as a last chance is to put one or two inside of the MCT. If you walk right up to it and crouch, you can usually place a couple and they can't be seen. It helps catch anyone that might have made it through the main mine areas. Some levels, though, are very tough to mine because of the number of critical areas to cover.