they are 100% safe. the problem is, most are way to large for most vivariums. my N. ventricosa throws pitchers that are about 8" in length. there are some miniature cultivars, but who wants to plop $150 or more on a plant in their viv? even i am not that crazy (although i have dropped $100 on one plant once). the upside is that you can buy a juvenile plant relatively cheaply and since they are such slow growers, have it for a long time before it out grows the tank. here's my N. ventricosa and you can see how large of a plant it is:


as far as smaller frogs being caused harm by them. it's not going to happen. a lot of people seem to think that there is some sort of dissolving acid in the pitchers, but in reality, it is just water. enzymes from the plant mix with the water to dissolve insects that get trapped and die in there. some insects actually live inside pitchers. they feed on fallen insects, even lay their eggs in the water. i have seen people use nepenthes in dart tanks and the darts hung out in the pitcher, just like they would a brom. so you see, they are frog safe.