Rahu in the 6th in Sagittarius

Rahu in the 6th house in Sagittarius means the 6th sign is Sagittarius, so the lagna is Cancer. Ketu occupies Gemini in the 12th. Jupiter rules Sagittarius, and for Cancer Jupiter also rules the 9th, so the sign lord of this Rahu is the 6th lord and the 9th lord together. The node of competition sits in the fortune lord's sign. That is a mixed dignity of topics, conflict and dharma sharing Jupiter, not a simple worsening.

The 6th is an upachaya. Rahu there is one of the placements generally allowed to work: service, opponents, debt, and the will to answer them. Calling the mahadasha worse ignores the house type.

Rahu mahadasha lasts 18 years and will emphasize this 6th-house node: disputes, employment under pressure, health routines as work rather than as a diagnosis, and competition. The antardashas still run Rahu, Jupiter, Saturn, Mercury, Ketu, Venus, Sun, Moon, and Mars. Rahu-Jupiter is important because Jupiter rules the sign and also the 9th. That slice can turn a fight toward a teacher, a law, or a long journey. Rahu-Ketu brings the 12th-house node into the same months, so expense and foreignness visit the competition.

Mercury rules Ketu's Gemini and, for Cancer, rules the 3rd and the 12th. The axis is courage-and-expense opposite the 6th.

The period is harder if Jupiter is debilitated in Capricorn, because the sign lord of Rahu is fallen, or if the Moon is conjunct Rahu, because the mind sits in the 6th under the node. It is not harder merely because Rahu and the 6th and Sagittarius were named in one question. Mars is yogakaraka for Cancer. A Mars period may be kinder than this Rahu period, and that comparison is about lordship, not about Rahu being cursed in every 6th.

Read Jupiter's Navamsa. A strong 9th-and-6th lord makes the 18 years a contest you can use. A fallen one makes the contest sloppy. Worse is that difference.