Jane lpblackmesa As long as I remember, since marsmallow, android develop new secure environment that called AVB (Android Verified Boot), one of the secure feature implementation are dm-verity and FDE(Full Disk Encryption). DM-Verity have more advanced developing to ARB (Anti Roll Back) protection. All of this is to support secure platform called COT (Chain Of Trust), which begin from PBL inside SoC untill the platform bootloader, and continue by AVB (Android), from Little Kernel to their System.
Short story short, in qualcomm, they have independent block that managed this secure and fused area called QFPROM, this block keep the certificate and related key that hashing and signing code, and binding with rpmb inside emmc.
In xiaomi, specific to EMMC Replacement, the problem is begin from Redmi 3S, which already use Marsmallow as their based system. And the problem continue into next generation after this model.
Luckily, in Marsmallow and Nougat based system, above DM-Verity and FDE still able to bypass by patch their boot image that combine with litle kernel from engineering firmware. But unfortunately, in Oreo based system Google start treeble project, and move DM-Verity to system, which cause difficult for us to bypassed it like in previous based system.
All of the above difficulties only apply to the phones with Lock Bootloader. If the Bootloader already unlock (Official Unlock), means the COT is already broken, then emmc replacement will be just like generic platform.