The problem with string theory is its lack of predictive ability. The equations have so many arbitrary parameters that it can model basically anything.
M-theory has been around for a long time (since 1995). It constrains some values forthose arbitrary parameters, but the model still doesn’t have any real predictive ability. It’s a nice mathematical framework, but that’s all it really offers.
It’s like saying “climate change is a scientific theory”. Ok, so climate will change. Granted. So from that hypothesis tell me what you the mean world temperature will be in 50 years from now. String theory has the same problem.
I don't see how this is an issue. The established method for dealing with such difficulties is to declare a set, S, of all such arbitrary parameters, and then label set S "Dark Matter."
See? Problem solved.
It also lacks any known (at least to me) method for validating the theory. Like the book on the subject states: It's not even wrong. Let alone provably right. It's neither. It just IS.