Cavalier crystals.
Firstly, let me be the first to admit that I was taken in by the hype surrounding the Cavalier crystals -- in no small part due to multiple videos of great pulls. However, having opened a decent number of them, I think I can safely say that they're not all that much better than any other crystals.
Granted: the price tag of them is fair, and the 50% chance to get something other than a 3* is nice. However, if you're expecting to get regular 5* pulls, or you're trying to get a specific character, you're going to be disappointed. These crystals are no more useful for such endeavors than are PHCs or FGMCs.
So, buyer beware: your luck with these, just as with all other crystals, is more likely than not going to be bad. Though, you at least will be able to take comfort in the fact that you won't have wasted as many units on these as you would've on others.