What Appearances must the witches avoid at all costs?

I'm not sure what you mean by "appearances". Can you tell me what you mean?

I suppose witches' powers are limited so they need to appear prophetic and scary. As one witch says about the curse on the sailor, "Though his bark cannot be lost,. Yet it shall be tempest-tossed." They are also quick to give credit to their evil masters (spirits...) rather than claim they are in full control lest they anger higher powers.

I mean by not showing the witches' weaknesses, are they had something bad or something that can cause trouble by showing to other characters? Because they're witches, Im not sure that they have any weakness at all, especially in this kind of story

Hi Sara, the witches have certain limitations. They can't actually kill anybody. They can play with spells and tricks but they are limited to tampering with the elements and minds of weak people like Macbeth. If you recall, I think the first witch, is pissed off at a sailor's wife. She tries to get at her husband but in a roundabout way. She can't destroy his ship, "though his bark cannot be lost", but she can mess with the weather and make the ship, "tempest tossed".