I never really cared about this criticism since I love both series, but I will say that later installments of the Critters series had pretty clear Gremlins influences, like that Hungry Heifer restaurant scene in Critters 2 or the kitchen scene in Critters 3 where one of the critters drinks dishwashing liquid.
The first Critters movie has very little in common with Gremlins, though.

This is one of the biggest bandwagon myths out there. Critters didn't copy Gremlins. They were just conveniently released around the same time with a similar antagonistic creature. Critters was in production first, like you mentioned, and actually had elements removed from the script to soften the similarities between the two series, yet people still rag on it for being a rip-off. It doesn't help that Gremlins is by far the more popular of the two, while Critters has a cult following but nothing mainstream, so it's much easier to give Gremlins the leg up.
Cena doesn't overcome any odds. He's a wrestler with a God Mode cheat on.