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Vertical integration increases payload flexibility. Stacking on the pad obviates the need for large crawlers, heavy load capable crawlerways, and VABs. A disadvantage of stacking on the pad is lower cadence.
Landspace uses a hybrid system where only the payload & fairing are stacked on the pad, retaining the benefits of vertical integration. The new Long March 8/8A pad at Wenchang also uses a hybrid system where the boosters are attached in a separate mini-VAB before stacking on the pad begins.