Here's a rough worry about the view. It doesn't deny that the belief that, say, this glass contains water can be justified on the basis of your experiences. It insists that its justification is provided by phenomenally individuated states which are supposed to be common to subjects on Earth and Twin Earth. Here's the objection to this proposal:
P1. To have sufficient justification to believe propositions about the external world, these propositions have to be more likely than their negations on the evidence one has for these propositions.
P2. If these propositions are the contents of beliefs with wide contents, phenomenal mentalism implies that these propositions will not be more likely than their negations on the evidence one has for these propositions.
C. Phenomenal mentalism implies that one cannot have sufficient justification to hold beliefs with wide contents.
The first premise says that you cannot justifiably believe p if you need evidence to believe p and the evidential probability of p is not greater than the evidential probability of ¬p. I take this premise to be eminently plausible. To deny it, it would have to be possible to have beliefs about the external world that were not more likely to be true than not on one’s evidence. It’s not at all plausible to deny that.
Let 'w' be the proposition that the glass contains water and 't' be the proposition that it contains t-water. Now, we might suppose that our subject only grasps the concepts to entertain w but the grounds she has for believing w are the same grounds as the grounds her counterpart has for believing t.
The second premise says that the evidence you have to believe this glass contains water (‘w’) is the same evidence you have to believe that this glass contains twater (‘t’). The evidential probability of w cannot be greater than .5 because it is equal to the evidential probability of t and w and t are incompatible. (Indeed, since the evidential probability of the disjunction of t and w isn't 1, it will be less than .5.) Even if these exhausted the possibilities (which they don’t), w wouldn’t be more likely than not given your evidence.
Might the phenomenal mentalist try to avoid this worry by arguing that probabilities are only defined relative to propositions that you grasp? I suppose that's a possibility, but it seems like an odd one. Without going this route, it looks like the security of the grounds for our beliefs about the external world undermines the thought that these grounds are sufficient.