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You can unlock the bootloader and install whatever you want.
At the end of the day, Miui will give you stability, longer battery life and its developed by many, many people....professionally trained people. While custom roms are sometimes fantastic, they are developed mostly by individuals and occasionally small collaborating groups, and too often these roms are made in the developers spare time and in some cases without the complete source code.In result, some of these roms,can be not so great and have issues. For example, custom ROMs can be unstable, run too hot and much less battery efficient,and occasionally causes bricks However,as I said some roms are fantastic. In truth, many of these roms are made by students, who do it for free as a hobby while the stock rom is going to be more streamlined and much more likely developed by a credentialed graduate who gets paid for their work.
My daily driver is Miui, my daughter likes AOSP, so I bought her an A3 with Android One and she loves it. By all try what's available for your device, just be careful and research the ROM and it's installation steps carefully before installing.