This is mostly accurate. Nintendont and USB Loader GX both support loading GameCube games from SD in a folder setup similar to the USB set up. SD:/games/Title [GAMECODE]/game.iso. See the Installing Games Manually section of my stickied vWii guide if that folder setup is fuzzy for you.
As far as SD limitations, the Wii and Wii U only officially support SD and SDHC cards, which go to up to 64GB, but some 64GB cards are SDXC, so if you want to play things safe, go for a 32GB card or make sure the 64GB you buy is SDHC. However, Wii U can use most SDXC cards (up to 2TB) that are thrown at it, and there has been a user able to get vWii to read up to 256GB SDXC, so it appears some SDXC cards work.