Name of the fallacy: "your argument is overly simplistic"

Question

The structure is as follow:

• Person A states argument X.
• Person B says that things are not so simple to discredit argument X. A special case is when B argues that: "this-is-not-about-D", pretending that person A is using a false dilemma D.

Of course, B's argument could be completely legit or right, but not always:

• (A) Sometimes B doesn't properly explain why X is overly simplistic, insufficient or a false dilema.
• (B) Sometimes B presents a falsely related fact Y to confuse person A and then state X is too simplistic because he didn't consider things like Y.

Case B can be confused with two well-known fallacies, but I don't thing it qualifies as such:

• The introduction of Y is not a straw man fallacy because person B doesn't argue against Y as a way to argue against X, because person B doesn't argue against Y at all.
• Y is more like a distraction, but I don't see it as a red herring either because the distraction Y is not used to avoid arguing against X, rather, is to confuse person A and then argue against X without justification because person A is too confused to argue back.

Has this fallacy a name?

Answer 1

Broadly speaking, we can break this into three cases:

• The argument X is a (purely) deductive argument - if the premises of X are true, then the conclusion of X logically follows from them. In this case, we may either attack the argument as invalid (i.e. the conclusion does not actually follow from the premises) or attack the premises as untrue, but either way, bringing in additional facts accomplishes nothing, so that's a non sequitur fallacy. However, if your interlocutor responds with such extraneous facts, they may have mistaken your argument for an inductive or abductive argument, so you should apply the principle of charity and strive to clarify that you really are presenting a pure deductive argument from known or admitted premises.
• The argument X is (primarily) inductive - that is, it presents a pattern of facts and tries to conclude that the pattern will continue or be repeated elsewhere. In this case, it is entirely valid and justifiable to bring in additional facts which break the pattern in order to refute X - the "pattern" may simply be a fluke, and bringing in more examples may show that it doesn't really exist. For example, if someone is trying to argue that a specific economic or political policy tends to produce good results based on some particular set of examples, it is valid to point out other cases where that policy did not work as well. However, this assumes that the fact is actually relevant; if it has nothing to do with the pattern, then it's just a non sequitur. In a political context, it's sometimes called "whataboutism" because these fallacies usually take the form "What about X?" where X is some unrelated or tangentially-related fact.
• The argument X is (primarily) abductive - that is, it presents a set of facts, and tries to identify the most straightforward explanation for those facts. In this case, identifying facts which are inconsistent with the conclusion of X is not only valid, but it may be the only effective way to refute X (that, or identifying a simpler alternative to X). For example, if someone is arguing in favor of Newton's law of universal gravitation because it's the simplest explanation for the planetary orbits, it is valid to bring in the orbit of Mercury as a counterexample where Newtonian gravitation provides the wrong answer.

Since you have told us literally nothing about X or Y, I can't identify which of these cases you're talking about.