Reaching. It’s best move from the core, not the arms. Instead of reaching the arms outward, draw them into the shoulder joints for better stability as well as safer ranges of motion.
Rounding. Rounding the back creates poor overall posture, weakens the back muscles, and places the spine in a more vulnerable position.
Rolling. The shoulders (and the arms) are on the back body and the chest is on the front body. These positions should remain in the final yoga posture. Rounding the shoulders makes them vulnerable to rotator cuff injury and significant joint damage.
Of course, when moving into postures, there are times that some reaching, rounding, and rolling occurs. But if the intention is to complete the pose without these actions, the postures will be deeper and safer.