If one doesn’t understand a topic, what value is there in what one says about it?

That sounds like a mere dismissal, but let’s try to formulate an answer to it as an actual question.

For one thing, if a field is full of intentionally confusing/obfuscated nonsense, then hopefully one could legitimately refute it without understanding the nonsense. For example, if some variant of astrology, or some theory of literary criticism, is intentionally so difficult to grasp, that none but it’s advocates have taken the time to understand it, that should not prevent the field from being dismissed as pointless nonsense.

But what is the grounds on which such a dismissal should be based?

Well, in the case of astrology, you can find that whatever predictions it makes are not better than chance. You don’t have to understand how the theory works to evaluate this, you just need the predictions made by the practitioners.

In the case of a nonsensical theory of literary criticism, either it does not make any claims which have a meaning that can be evaluated outside of itself, in which case it can be dismissed as not saying anything that any non-practitioner would care about, or it does, in which case its merit can be judged, in part, based on those claims.

But suppose there is a theory (or related family of theories) which one finds too complicated to understand, but which the community around it(/around them) uses to make accurate predictions.

Can one then potentially offer useful criticism? In some cases, yes! If there is another theory which can be seen to make predictions or judgements which are just as good, or better, and such that one can give a strong argument that it is simpler, then one can put that forth as a good criticism.

But, note, one should have a clear idea of what one means by simpler, or at least, of the basis by which one concludes that the other thing is simpler.

Are there other cases that work? I think so, but suspect they are all, fairly limited in scope.

If a community is able to make predictions well using a theory which has few parameters (in an appropriate way of counting parameters. E.g. no splitting the digits up to turn two parameters into one.) and no community has another theory which makes comparably good predictions, I think that without understanding the theory, or at least without understanding some part of it well, one wouldn’t have many compelling avenues of criticism.

(If one is able to understand some small part of it well, one can potentially demonstrate that that small part of it seems to be e.g. obscurantism around a trivial point, or other issues with that small part. If one does this with a handful of very small parts this could be a quite compelling criticism, especially if no one is able to give a good justification for why it is said like that.)

Of course, one can think all one wants about whatever topic. There is nothing objectionable about anyone thinking about a topic.

But in presenting one’s thoughts, one should express an appropriate level of confidence based on how what one knows compared to what others know, about it.

When people (not like you. I’m not talking about you here. I’m thinking of comments I see on youtube videos) with no math background think that “quantum mechanics is too complicated. It must, therefore, be wrong.”, they are making a mistake.