Compared to the iME, not much, since at least the secure enclave subsystem won't run any non-Apple code. The scary difference about the iME is that it is directly connected to the network.
The difference wrt. Apple Silicon is, that AS firmware blobs run on separate chips and 1) cannot access the main memory freely; they are gated behind IOMMU and 2) there's no SMM equivalent for any of them, so the main CPU time cannot be stolen by firmware.