Short Long answer would be discussing the rise of pop astrology which is essentially capitalism finding a way to profit.

https://www.astrology.com/retrograde

Based on that common definition a long short answer would be that a retrograde mercury's energies are different than the typical form causing disruptions. Mercury's association with perception, communication, electronics, short duration travel can bring one to the conclusion that a disruption in those areas could be bad.

If you were to examine the transit chart containing the retrograde planet in context of one's natal chart you would be able to get a more useful picture. For example someone that had a mercury opposite Venus in 7th house may have issues communicating in relationships but when experiencing a mercury retrograde simultaneously as a venus/moon conjunction it may be the catalyst for you to communicate your hidden emotions to a partner, the disruption could literally cause one to break a shy cycle and finally ask someone out.

Hope that answers your question, please feel free to follow up if it didnt