I've had smaller crypts growing for varying lengths of time in sand, then Flourite, and right now I'm growing a new, cute little species called C. pygmaea in Eco-Complete.
I had a bunch of generic crypts (some sort of wendtii) that I started off in sand substrate with very little fertilization. The plants quickly recovered from melt just fine and looked okay. They were mostly green and had long petioles (is that what the leaf stemmy thingie is called?). I took a few of these and planted them in Flourite in a tank with a little more lighting. After a few weeks, you couldn't tell that these were the same plants. It tripled (at least) in size and turned a deep reddish-brown color. The petioles were very short, making the plant look "fuller." Very impressive. Every few months, though, I'd notice that the crypts would go through a major melt, which was not pretty to look at.
I have two pieces of C. pygmaea in Eco-Complete and one in a sand-bottomed tank. The two in the Eco-Complete went through *complete* melt and are growing back leaves relatively quickly. The one in the sand-bottomed tank retained maybe two leaves but only one new leaf has sprouted and it's very slow to unfurl. OTOH, it may be a lighting issue. I definitely think that a rich substrate is key in growing out beautiful crypts. Root tabs would probably be a good alternative, though, if the plants are already in sand or gravel. I've never tried it, though. I know they like Jobe's spikes, but they would probably like iron-containing stuff even more.
-Naomi