The antonyms for "considered opinions" could be impulsive thoughts, unexamined beliefs, or rash judgments. These are opinions that are not carefully thought out, but rather made quickly and without reflection. They may also be opinions that are based on incomplete or inaccurate information, leading to flawed reasoning and faulty conclusions. The opposite of a considered opinion is one that is based on little or no evidence, or that is formed without taking into account all relevant factors. Such opinions can be risky and potentially harmful, both to the individual holding them and to those affected by their actions.