The Yandex-engine is only able to give a first impression of what the translated text should look like, it is not anywhere near an acceptable text. It could be wise to block publishing a text translated with this engine unless it has been altered sufficiently. Not just altered with a keystroke or two, but several words should be moved or otherwise altered. How many such alterations would depend on the size of the text.

Translations between closely related languages by the Apertium engine sometimes creates texts that don't need refinements, so an edit limit needs to depend on both engine and language.