Some of these differences are intentional; partly to align with LE standard namings, but also because we (me) don't want users "updating" from all kinds of unknown legacy images to AMLGX and then showing up with tricky-to-solve support issues. Boot is already "fun" before you factor in large Kodi major version bumps and the Python2 to Python3 change which causes add-on carnage.
After digging through the whole u-boot process, aml_autoscript, etc. I can fundamentally see why you wouldn't support eMMC's at all on Android TV-boxes and not even touch them. It's a nightmare, you would have to format & partition the eMMC to be supported by mainline, you probably would have to replace u-boot, and it just starts piling up with bricking scenarios.
I have only worked up the nerve to chainload mainline u-boot on the Vero4K, pretty reluctant to format the eMMC and load that mainline u-boot. And naturally all working from SD cards. It's just not worth it, so far I haven't seen an SD card failure, and I have weekly backups anyways of /storage.