I feed pellets for both practical and experimental purposes.
The pellets that I have been offering are from a batch made for a colleague of mine to feed to Lake Sturgeon. I have willingly accepted any risks from such an experiment, none noticed so far.
The pellets also offer greater economic management of my rays. I can get them in bulk (40lbs/time) portion into smaller containers and freeze for long term (relatively) storage. Pellets tend to be more stable in the system. Less fouling. Less undigestible matter excreted and clogging the coarse filter media, less filter changes.

One more pro-pellet comment is based on quality. You do not tend to find disease organisms or pollutants in manufactured pellets.
When the Smelt run up here I harvest about 120lbs sort into groups of a dozen and freeze.
Nightcrawlers are seasonal as well. If it rains I am out pickin' crawlers.
Frozen shrimp I buy as the mood hits me.
Variety is critical to health, yes, but I vary their diet over the course of 12 months, not necessarily per feeding.
I vote pellets.
Respectfully submitted, FireMedic.