After a long experience in astrology, I would say that the first and often the biggest problem is that there are unfounded arguments, opinions turned to knowledge, traditional proverbs, even in the interpretation of planetary aspects. Somebody somewhere sets in motion a nice-sounding interpretation of one or two sentences, and it starts going around the world. Nobody questions it for a long time, if ever.
Ok, here are my own two sentence simplifications of Mercury aspects:
Mercury/Sun: it helps create convictional speeches and opinions that are presented in weighty and authoritarian ways. Someone who has seen the truth.
Mercury/Moon: knowledge, information flows in and out, diverse interests can get in the way of focusing on something in depth. Learning abilities are often good, sometimes excellent.
Mercury/Venus: writing and speaking are looking for a beautiful form, a subtle way of saying things. Often the sign of a very talkative person who likes to entertain and amuse others.
Mercury/Mars: Traditionally known as the sign of the critical mind. Skeptical, pushy and averse to complexities. Straight talk and argumentation can turn into critical aggression.
Mercury/Jupiter: The need to gain appreciation for one's mindset by pursuing academic degrees or accumulating knowledge for one's library. The person wants to speak and write about important issues.
Mercury/Saturn: A serious mind that wants to gain certainty about things and about what is true. Sometimes the mind is very abstract, in which case mathematics and logic, for example, become important. Science is based a lot on reduction and that’s where Saturn is powerful.
Mercury/Uranus: Interested in "heavenly things", space, astronomy, astrology – yes they are half-brothers – new technical inventions and technological advancement. Sometimes illuminating mind-set, but it depends - well, of everything else.
Mercury/Neptune: At best in creative pursuits where you can use your mind freely relaying in your imagination intuitive flashes. Often indicates a mind that understands symbols, metaphors, allusions and the power to enhance the world by verbal images – poetry.
Mercury/Pluto: There is power of coercive language, sometimes manipulative techniques to get the knowledge you want. The mind is able to dive deep into psyche’s subconscious processes. This all can be used to one’s own advantage, even to unscrupulously using other people or to unravel the unconscious secrets in order to help to heal psychological wounds.
Whatever we say about how a certain aspect works, you never “get there”; there are countless variations how the aspects express themselves in a particular chart. That’s why astrology never tells “the Truth”. Sure, it opens up endless series of possibilities, which can manifest in the structure of human nature and character, but not with absolute sense. Astrology is a science of uncertainty. Every detail in the chart (such as Mercury/Uranus aspect) depends on everything else in the chart. Therefore my motto is: The context rules.
You like numbers? Which is more conducive to astrology, Mercury/Uranus aspects or Mercury/Saturn aspects? Here’s Mercury/Saturn and Mercury/Uranus main aspects in a tiny database (129 items) consisting astrological professionals, students and amateurs alike.
Mercury/Saturn: 0= 6, 45= 4, 60= 9, 90= 7, 120= 16, 135=8, 150= 7, 180=4, in total: 61/129
Mercury/Uranus 0= 4, 45=6, 60= 6, 90= 12, 120= 8, 135=8, 150= 5, 180=11, in total: 60/129
Orbs: 0,180,90,120 = 8 degree, 45,60,135,150, 3,5 degree.
Forgive my existence, sorry taking space on this planet.