Suppose Singers is right that all beings that have the ability to experience pleasure and pain should have equal moral consideration. But this theory does not fit in some circumstances in real life.
For example, for some people who have a disease called analgesia, those people cannot feel pain even when their leg is broken or during the heart attack. Based on Singer's theory, those people are not worthy of equal moral consideration as others since they cannot feel the pain, which is absolutely wrong. Moreover, using sentience as the only criterion to value whether a being worthy of moral consideration is not thoughtful. Sentience is a subjective concept. When we are saying that dogs can feel pleasure and pain, we are just applying our understanding of sentience to dogs. Perhaps, in the dogs' perspective, their sentience is not the same as us, but since we are not them, we are unable to know their feeling and whether they have those sentiences.