Synastry is an analytical method in Western astrology for assessing compatibility between people/companies/countries, etc. It is applicable to all kinds of “relationships”.

However, like any other information (from any source), synastry is not a verdict. Always listen to your heart and don’t let others dictate your life.

Synastry is most practical for partners who are already established as individuals, mature, and know their own and their partner’s quirks.
Let’s say such a “couple” faces a conflict or a controversial situation, and here synastry helps to see the whole process from an alternative perspective, as the situation itself could be cyclical, a result of individual programs, karmic contracts, or prognostic aspects (world’s darling retrograde Mercury).
There is no point in applying synastry to people who have just met, especially for romantic relationships. Many people thoughtlessly put the responsibility for their lives onto the indicators in their natal chart, which is more likely to harm them than to help.
First, get to know people yourself, and if you feel that someone is your “the one and only”, but inexplicable conflicts arise, you can use synastry to identify the underlying issue.

If it’s about a business partnership, you should also soberly evaluate the situation and find a balance between someone’s advice, logic, and your own intuition. Let’s say an astrologer tells you and your partners could have different opinions on some matter that’ll cause a conflict. In this perspective, this information will be useful. But if you are told: “Oh no, run away! They’ll destroy you, take all your money and luck!” – You’d better RUN away from such an astrologer.

A competent practitioner will always offer you options on how events might unfold, what compensatory methods can be applied, and so on.

Therefore, synastry is a powerful analytical tool, but it is not a panacea or a law that absolves you of responsibility for your own life.