They wouldn't, as it is a genjutsu that alters the terrain, the spikes would do no more than phase through the target which would have them come to an obvious conclusion of it being a genjutsu. The genjutsu itself is used more to intimidate, confuse or/and distract the opponent, using the spike example again, it would be used to distract the opponent as they see the spikes emerge from the ground leading them to try & counter it, while they do the user can then strike the opponent as they deal with the genjutsu.
A question.. When the spikes do emerge, I am guessing they would not create any vibrations across the ground due to it not actually altering the terrain or even make any sound, but would the opponent see the ground around the spikes crumble apart as it emerges or would it just look like it phased through the ground.