I wish folks would stop conflating boot media and root media. There's no reason your SD card has to consist of anything more than u-boot.
SD is fantastically simple, so that the boot rom can get at a bootloader without much effort (generally the bootrom just looks at a memory offset on the mmc device). Once you start speaking newer faster protocols, this simplicity is lost. You're not likely going to find a bootrom that implements all the bits needed to get u-boot from a sata device.
In a perfect world, once these are no longer developer devices, the mmc would be replaced with some spi flash (or even an emmc) with just u-boot.
On 90% of embedded dev systems, you're better off thinking of the sd card as a "bios chip" then a hard drive. The fact you can also use it as a block device to store a rw filesystem is almost incidental, and should probably be avoided.
SD cards speak SPI. In fact SPI is all SiFive supported on their first 2018 HiFive Unleashed. As you say, it's good enough to bootstrap. That old board has gigE. Once you're up enough to TFTP you're away. I've never actually bothered to set that up on mine -- I boot a full kernel with NFS support and then switch to that.
SD is fantastically simple, so that the boot rom can get at a bootloader without much effort (generally the bootrom just looks at a memory offset on the mmc device). Once you start speaking newer faster protocols, this simplicity is lost. You're not likely going to find a bootrom that implements all the bits needed to get u-boot from a sata device.
In a perfect world, once these are no longer developer devices, the mmc would be replaced with some spi flash (or even an emmc) with just u-boot.
On 90% of embedded dev systems, you're better off thinking of the sd card as a "bios chip" then a hard drive. The fact you can also use it as a block device to store a rw filesystem is almost incidental, and should probably be avoided.