What's the downside of the HAL pattern, and how do you avoid over-abstracting?

Every abstraction adds indirection and reading overhead, and the wrong kind adds runtime cost (virtuals' vtable indirection) or code bloat (many template instantiations). The most common failure is over-abstraction: building an interface that mirrors every register field and config bit, which isn't an abstraction at all, it's the hardware with extra layers, harder to read and providing no portability benefit because it's still chip-shaped. You avoid it by keeping interfaces minimal: expose only the operations the application actually uses (set/clear/read), hide the registers, and add abstraction only where you need portability or testability, not reflexively on every peripheral. Also choose the cheap mechanism: prefer compile-time (template) polymorphism so there's no runtime cost unless you specifically need runtime swapping. The best HAL is the smallest one that lets your logic ignore the chip; anything beyond that is cost without benefit.