Any remote-attestation scheme is theoretically vulnerable to attacks where the CPU manufacturer includes backdoors in the processor hardware (either deliberately, accidentally, or under compulsion from a third party).
Intel's implementation is considerably worse than that. Even if you assume the hardware itself isn't compromised, every remote attestation has to go through the "Intel Attestation Service" which has no end-to-end protection. The IAS is what actually validates the enclave's signature, and it returns a "success" or "failure" message which is signed with an Intel key. But there's absolutely no technical measure that prevents Intel from being compelled to sign a falsified response; a client would have no way of telling the difference.
This is documented by Intel  and I'm hardly the first to notice it  but people still seem to talk about SGX as if compromising it is equivalent to backdooring the CPU, which is inaccurate.