In any case, there is no evidence in the multiplicity of different testimonies that these are hallucinations. If aliens had come to Earth several times, they would probably describe it in completely different ways: one would see a sea, the other a forest, and the third a large city. Is the conclusion their friends were supposed to draw that the earth is nothing but a hallucination?
Another critique would argue that there are people who “see” many things that are difficult for us to accept as real, such as aliens, ghosts and so on, and that according to the above argument we will have to treat all of these things as true unless proven otherwise, which seems unacceptable.
The answer to this is that each case must be examined on its own merits. In many cases, it turns out that people who testify to such things have not really seen them clearly, or that they are not mentally healthy, or that they are lying for some reason, and so on.
In many cases the same people themselves will later recognize that they were wrong, or that it was their imagination. But imagine an intelligent and sane person who testifies to things he has seen clearly, and he usually knows how to distinguish between imagination and reality, and is convinced even after a while that what he saw is real, and the things he describes do not unequivocally contradict our knowledge of reality. His testimony at least is a reasonable possibility.