Portability: one you hand-code assembler for an architecture you lose compatibility for others. Unless you provide multiple code paths to account for this, but

Portability: one you hand-code assembler for an architecture you lose compatibility for others. Unless you provide multiple code paths to account for this, but that increases testing and maintenance requirements. Future optimisations using better on-chip features, or other new techniques: new instructions that reduce the task might get used by the compiler but it won't update your section in hand-coded assembler. Maintainability: you don't want to force larger parts of your workforce/community to be familiar with assembly if it is not useful day-to-day, or lock your assembly capable people to maintaining those hand-written-assembly portions.