Ascending quickly (e.g. on a flight or trying to reach Ladakh in 1 day) will lead to shortness of breath and even acute altitude sickness. When on a tour with Himalayan Ecotourism, you will ascend at a pace which is comfortable and avoids altitude sickness.
It can get hot and cold within a matter of minutes. Wear layers of clothing.
Road conditions can be variable. In the monsoon, there can be landslides which can add hours or days to the journey. Even in the dry season, melting snows can turn placid rivers into raging torrents which can make passage impossible.