X's = household cleaning products
Y's = hazardous substances

Consumer: X's are not Y's, but many of them are harmful substances, especially if they are used improperly.

After replacing the pronoun:

A) X's are not Y's, but Y's are harmful substances, especially if they are used improperly.
B) X's are not Y's, but X's are harmful substances, especially if they are used improperly.

B makes more sense as the "Consumer" is trying to tell something about X's and logically additional information should be about about X's and not Y's.

Therefore, "them" does not refer back to Y's and there is no ambiguity.

This may be a little hard to digest but let me know
whether it makes sense or not.

I understand you are indicating that as long as you can make sense out of the meaning of the sentence, there may not be an ambiguity.

But It still does not give me clear idea.


Could you explain with another example where there is an ambiguity and the logical meaning of the sentence is changed ?