Sardines have numerous kashrus issues to be concerned about, among them;
Crustaceans, krill in the cans of sardines.
Anisakis worm in the gut area.
Skinless, when a mashgiach can'y physically check each fish.
"Kovush", It's common for other non-kosher species to be in the net together with the kosher species. By sardines, they go through a multiple sorting to remove the unacceptable species. In the meantime the sardines were in the holding tanks with non kosher species more than 24 hours "kovush".
With so many serious kashrus issues many have discontinued consuming any sardines at all. Others are waiting for an all encompassing "HETER", which may even distort some facts in order to proclaim a "heter", and degrade all others that consider it not acceptable.
Rav Eliyashev, Shlita said one may not rely on those "Heteirim".