If you choose SD-USB installation, the image you can flash to SD card the same as Armbian images.
If you choose EMMC installation, you can flash the image to eMMC var Amlogic Upgrade tool.
You can find the images in directory like:build/images/VIM1_Ubuntu-server-xenial_Linux-4.17.13_arm64_SD-USB_V20180912.img
About hw acceleration, does this help with watching chromium videos?