uh...? I've got most of if not all of these No-Intro sets, and a compressed (.7z) backup of them is almost 50GB, raw being 60-80GB, can't remember off the top of my head. The DMG and GBC sets are well under 4GB.
(also a lot of those are outdated as hell, so there is that...)
As for the SD argument, the onboard hardware is an FPGA of some sort emulating MBC, ROM, SRAM, etc. so that it can read/write files to/from SD. There's no extra "taxing" here. The hardware variance might be an issue, but they're pretty close to each other internally (being standardized microSD cards), so it shouldn't be too hard to include SDXC access, if it isn't handlable by SDHC code (is it "code" in terms of an FPGA?) by default. If I limit partition size, that should be all that would matter, since the filesystem would be the proper size and empty space should be ignored...
then again FPGAs are hard to work with so i dunno