Many skeptics say theories that contradict materialism are unscientific because those theories are not falsifiable.
For a theory to be scientific, it must be testable. For a theory to be testable, it must be falsifiable: there must be a situation, where if the theory is wrong, you can demonstrate it is wrong.
For example, you can test the theory of gravity by measuring how objects fall. If they don't accelerate the way the theory of gravity predicts they should, then the theory is wrong, it is falsified. If objects do fall the way the theory predicts, then the theory is right.
Skeptics often say belief in spirits or psi is unscientific because any unexplained phenomena can be said to be caused by a spirit or by psi and there is no way to disprove it.
What may be surprising to many skeptics is that Karl Popper, who first proposed that falsifibility is necessary for a theory to be scientific, did not believe in materialism. He believed in dualism which holds that the mind is nonmaterial.
The wikipedia article on Karl Popper explains falsifiability:
Logically, no number of positive outcomes at the level of experimental testing can confirm a scientific theory, but a single counterexample is logically decisive: it shows the theory, from which the implication is derived, to be false. The term "falsifiable" does not mean something is made false, but rather that, if it is false, it can be shown by observation or experiment. Popper's account of the logical asymmetry between verification and falsifiability lies at the heart of his philosophy of science. It also inspired him to take falsifiability as his criterion of demarcation between what is, and is not, genuinely scientific: a theory should be considered scientific if, and only if, it is falsifiable.
The wikipedia article on Philosophy of Mind describes Popper as a defender of interactionist dualism espoused by Descartes.
Interactionist dualism, or simply interactionism, is the particular form of dualism first espoused by Descartes in the Meditations. In the 20th century, its major defenders have been Karl Popper and John Carew Eccles. It is the view that mental states, such as beliefs and desires, causally interact with physical states.
The wikipedia article on Rene Descartes explains that dualism as espoused by Descartes holds that the soul is nonmaterial and does not follow the laws of nature.
Descartes in his Passions of the Soul and The Description of the Human Body suggested that the body works like a machine, that it has material properties. The mind (or soul), on the other hand, was described as a nonmaterial and does not follow the laws of nature.
Is belief in psi or spirits unscientific? It depends. It depends on what those beliefs are theorized as an explanation of. For example, if you theorize that spirits are an explanation of mediumship, that can be tested. If a medium is communicating with a spirit, the medium should be able to obtain information about the spirit that medium could not otherwise know. If the medium could not obtain any information about the spirit such as their appearance, their personality traits, the things they did in life etc., then the theory that the medium is communicating with a spirit would not pass the test.
There is, in fact, a lot of evidence that mediums do communicate with spirits. The medium Mrs Piper passed many such tests.