Что обещание, что зарок - ненадежны

You can never be sure if someone will keep his word or not. See Обещанного три года ждут (O)

Cf: All promises are either broken or kept (Am., Br.). It's one thing to promise, another to perform (Am., Br.). Promises are like good piecrust: easily broken (Am.). Promises are like pie-crust, made to be broken (Br.). Promises are like piecrust: they are made to be broken (Am.)