From my perspective, the claim that sentience, coupled with the ability to experience pleasure and pain, is sufficient for moral consideration for these reasons.
First, if we admit this idea, we have to say that any life that can feel should receive the same moral considerations. However, it is really difficult to detect how much pressure or pain that an animal can feel. Thus we cannot say compare the different feelings from different life in the world. What's more, the feeling of an animal is dependent on the senses. what if an animal including humans that cannot feel? For example, a person is deaf which means he cannot hear anything. However, someone said bad word about him. Is that moral? I think that shows non-respect to the deaf man which is not moral. Thus, Singer is flawed.