A moon that is mostly water/dominantly water is terrible and the worst condition a moon can be in.
A mercury that is mostly fire and/or mostly water is terrible and the worst condition a mercury can be in.
Same can be said for the asteroids and planets that arent among that make up the zodiac signs. Such as ceres, vesta, pallas, lilith, etc.
The more watery and firery they are by nature follow the same logic when expected to our mercury and moon as the traditional planets and houses of our solar system do.
For example, lets say Ceres is conjunct moon (and for the sake of this example, lets just say Ceres is a water planet), and the moon it conjuncts is in a air sign, and has no other aspects. This would mean that this moon will be half air and half water.
Since conjunct = mixing two energies in perfect balance with each other and perfect harmony/perfect harmonious relationship with one another
Thats just my take on the worst aspects/placements possible.
Since it all starts with your personality, and your moon and merc determine what that is. And if your personality *****, it will be reflected through all you're other planetary configurations. Of course, there will be some placement to make up for it, as after all no one is created or born just talentless or giftless.
Thats why I was born with a terrible personality and bad looks by beauty standards, but I was born inot a very wealthy family and have been told by many women I have a strong sex appeal anyways to them.
I was born with bad looks by the standard of beauty (which is represented by venus) because my Ascendant is in Aries and conjuncts my moon (the sign my moon is in is pisces and its in the 12th house and is about 90% water of a moon). So my facial features are firery in character and watery too. Fire and water are the two elements least similar by nature to air. Venus is an air planet and an earth planet by character.
And aries ascendant gives the person a fast metabolism which makes me skinny for my age and weight (though in exchange for lots of physical energy) and im a male.
So I am very skinny for a male my age which is very unnatractive and also am wayyy hungrier than the average person because of the fast metabolism. I have a okay Mercury, but my moon is terrible. Mercury cant function right if the moon is weak.
Therefore my personality is very weak and I am very unnatractive by beauty standards, but I was born with saturn in taurus, saturn (limits, challenges, hardships) in taurus (finances, wealth) so you would think I have nothing right?
Well wrong, my saturn is well aspected so that I have a lot of wealth and inherited it too. This is reflected by how my Venus (which rules wealth, among other things) is in saggitarius (good luck, expansion, good things, benefits, happiness, prosperity) and the fact that its in saggitarius is reflected by how most of its aspects are harmonious.