The 9th house is generally considered to be the most benefic house being the highest of the dharma or fire houses, 1, 5, 9. It's the house of luck, fortune, divine grace, wisdom, one's spiritual path, religion, philosophy, teachers and gurus, progress, travel etc. When one starts the dasha of a planet ruling the 9th house, in the 9th house, or aspected by the 9th lord it can represent a smooth time period of good karma and many blessings.
However, there are some subtler modifications that also need to be considered. The 9th house as a dharma house is only one category of houses. The 9th house is also one of the apoklima or mutable houses - 3, 6, 9, 12, which are a weak category of houses. Apoklima houses generally have a difficult time manifesting in the physical world because of their mutable nature. They are the 12th house from the angular or kendra houses, the houses of power. The 12th house from any house can represent the loss of the indications of that house.
The 9th is therefore the 12th house from the 10th house, the house of career. It can represent breaks in employment or career, perhaps for ethical or spiritual reasons, or travel. This may seem contradictory or confusing because of the benefic status of the 9th house as a dharma house. It's a subtler point that needs to be considered.
Jyotish is a complicated science - it needs to be since we live complicated lives. It becomes very tempting to think in binary terms of black and white - this house is good, this house is bad. However, it's the subtleties that give subtle accuracy to a reading. By understanding the different categories of houses separately we can remember the subtle modifications. I agree that the 9th house as a dharma house takes precedence over it being an apoklima house or the 12th house from the 10th. It's generally the most beneficial house. However, you will see times that the 9th house acts less favorably and it may baffle you until you understand these other points.