IMO, retics are not any harder than the other rays..
They just are often abused because of their low initial price tag ($8 out of the river maybe), they are treated like any other fish..
Poorly shipped, over-packed in the bag.. hyperplasia, ammonia damage, internal parasites.. Many issues the rays could have before you even purchase them.
The other thing is retics, being a tiny species of ray, are in their 'teens' at just a few inches.. They have tiny little stomachs, with racing high metabolism, and small mouths.. They tend to be finicky eaters, but when they are juvie and growing, they need a ton of food.. Usually expensive live black worms..
This need to 'overfeed' them often results in compromised water quality, especially if people say "I have a 3" ray, it can live in a 50g!".. Space isn't as much an issue as water volume. You need to dilute the concentrated amounts of ammonia that FW rays release, in order for your bio-filter to break it down effeciently.. Work with a small water volume, and those ammonia spike concentrations will get worse.
So basically, make sure you get them from a quality supplier, QT'ed for parasites and shipped properly into the country.. after that hurdle, just feed the crap out of them and change the water very frequently.. I would imagine once they get past the 6" range and start feeding on prepared foods they are simple like Motoros..