I usually choose to ring the gong an elderly villager who is a master in two skills but is unlikely to live long enough to reach master in a third. If the gong grants wisdom he or she will likely become an Elder. If the villager is killed it is not really a great loss to the village, and it could be said to be useful if the population is maxed out, making room for another child to be born.

Probably the worst effect I've seen is when the gong takes all the stored food. However, with a large village it doesn't take long to build up stocks to a safe level by pressing all available villagers into action as farmers. Everyone gets unceremoniously dumped into the ocean to fish - it, too, could be said to be useful in that it helps train a number in farming.

Sickness, though it might mean a busy few minutes for your doctors, has its useful side, as it helps improve their skills. The loss of tech points is immaterial as I've only ever seen small amounts deducted and there's nothing other than a change of clothes for the villagers to spend it on anyway.

One gong result I've never been able to fathom out is when it gives/takes technology. I've not seen any difference either way.