I suspect that whether a western astrologer follows more modern or traditional methods is mostly a question of our personal intersections with astrological information available to us, and also of our own personalities and tastes. Which we would sort of expect: if we looked at one another's charts, could we predict what sort of astrologer each of us would be?
A lot of astrologers who hang out a professional shingle today really don't have credentials of any sort. I think this does everyone a disservice. But inadequate training and fraud have been going on since the days of Ptolemy (
Tetrabiblos I: 2. 6-7. In addition to criticizing the ignorant practitioners, he wrote, "...it is the same with philosophy--we need not abolish it because there are evident rascals among those that pretend to it.")
Modern astrology does have rules; as the basic primers on how to synthesize a horoscope will indicate. Maybe not so many as traditional astrology, but some are based on traditional astrology. Others are simply different. Just for example, in looking at planetary pairs in aspect and determining which one is stronger or weaker, the technique I learned was simply to weight them in order of distance from the sun. Thus tiny Pluto sits like the small child perched at the end of the see-saw, balancing out a larger child like Jupiter sitting closer to the fulcrum. Psychological astrologer Liz Greene has rules, and hers mostly slot horoscope elements into frameworks provided by psychological theory. I think in her writing, the moon is tremendously important as she subscribes (or once subscribed) to a thesis that "the bad mother" was responsible for most of the ills bedeviling the adult. Ruperti and Rudhyar popularized focusing on a planet's stage in its cycle around the sun.
Underlying most modern astrology is a meta-narrative about whether humans are primarily spiritual or material beings. Another meta-narrative is about whether the cosmos can have a deterministic and sometimes malevolent influence on human behaviour; or whether such a view is incosistent with the belief in a kindly God or at least a scientifically understandable, impersonal, neutral cosmos.
So maybe the principal way I would characterize much (though not all) astrology from the early 20th century onward: that oftentimes a philosophical or spiritual paradigm shapes the astrology to fit it. I suspect this was also true of former historical periods, but I'd defer to the experts on this one.
I would also think that anyone really wanting to get under the hood of Hellenistic, medieval, or Vedic astrology would be well-served by learning a lot about the social history of the era in which it evolved; as well as the periods' philosophies and theologies on the nature of human beings.
Just for example, in northern Europe in the Middle Ages, childhood and parenthood were very different than they are today. Infant mortality was huge, surviving children went to work young if they were peasants. Non-farm working class boys were usually apprenticed at a young age, and they lived with their masters, not their parents. Elite boys were also fostered out to learn the manly arts of hunting and warfare from a kinsman or neighbour. Girls typically married young; and their marriage was probably arranged if their parents had any status. Romantic love as we know it wasn't a huge option, apart from the late-breaking courtly love model of adultery. Nearly everybody was illiterate. Hardly anybody had a choice of work. There was hardly any middle class. The feudal class system was destiny in all but a very few cases. Depending upon the time and place, slavery was either perfectly legal, or something very close to it. Hardly any population growth occurred before early modern times because the death rate was so high. Life was often physically dangerous.
So I think one would have to understand this world in some detail, in order to contextualize the meaning of the sun and moon as one's parents, Venus/Mars as one's romantic/sexual nature, the notion of the MC as one's vocation, &c. I don't think we can assume without research that medieval people were "just like us" because our experiences are so different.
Also, I think any system of astrology has to be viewed with a healthy sense of skepticism. Each contains the good, the bad, and the ugly.
I could quote some real idiocy from modern astrologers. But then again, I read in Ptolemy,
Tetrabiblos III.13.159 of my Mars opposite Saturn:
"In the opposite positions he makes his subjects robbers, pirates, adulterators, submissive to disgraceful treatment, takers of base profits, godless, without affection, insulting, crafty, thieves, perjurers, murderers, eaters of forbidden foods, evildoers, homicides, poisoners, impious, robbers of temples and of tombs, and utterly depraved."
I don't think so. I've never robbed a temple in my life. Or maybe I've just missed my calling.