Developer Support Will Suffer

Custom ROM support probably won't be as good as previous Redmi devices which use a Qualcomm SOC since Mediatek can't rely on codeaurora as the base.

Unless of course mediatek starts releasing the full kernel sources.
But otherwise I'm guessing the Redmi Note 8 won't be as developer friendly as the old versions.
Will there be custom ROMs, absolutely but they'll lack the Polish that Qualcomm based phones enjoy with the modding and XDA community.