The MWI is qualitatively different from the other interpretations of quantum mechanics, although that’s rarely recognized or admitted. For the interpretation speaks not just to quantum mechanics itself but to what we consider knowledge and understanding to mean in science. It asks us what sort of theory, in the end, we will demand or accept as a claim to know the world. ...That's right. Ultimately MWI says nothing that you would want from a scientific theory. There are no facts, predictions, or confirming experiments.
What the MWI really denies is the existence of facts at all. It replaces them with an experience of pseudo-facts (we think that this happened, even though that happened too). In so doing, it eliminates any coherent notion of what we can experience, or have experienced, or are experiencing right now. We might reasonably wonder if there is any value — any meaning — in what remains, and whether the sacrifice has been worth it. ...
It says that our unique experience as individuals is not simply a bit imperfect, a bit unreliable and fuzzy, but is a complete illusion. If we really pursue that idea, rather than pretending that it gives us quantum siblings, we find ourselves unable to say anything about anything that can be considered a meaningful truth. We are not just suspended in language; we have denied language any agency. The MWI — if taken seriously — is unthinkable. ...
What quantum theory seems to insist is that at the fundamental level the world cannot supply clear “yes/no” empirical answers to all the questions that seem at face value as though they should have one. The calm acceptance of that fact by the Copenhagen interpretation seems to some, and with good reason, to be far too unsatisfactory and complacent. The MWI is an exuberant attempt to rescue the “yes/no” by admitting both of them at once. But in the end, if you say everything is true, you have said nothing.
MWI just says that everything that can happen does happen in some parallel world. It allows you to think and believe whatever you want. Probabilities are meaningless. Reality and facts are meaningless.
MWI is just the same as the child's fantasy. The proponents give the impression that it is a scientific theory that gives a detailed explanation of the worlds, with Hilbert space, wave function, Schroedinger equation, atomic forces, etc. Yes, but none of them can explain how all that apparatus tells you anything beyond the simplistic child's fantasy. There are no predictions or confirming experiments.
I used to to think that string theory was the epitome of unscientific thinking. But string theory is vastly more reasonable that MWI. String theory at least had some hope of getting some theoretical explanations. MWI explains nothing, and discards almost everything we know about science.