Brands should add a built-in cable when convenience, compactness, and the target device ecosystem justify it. They should avoid it when users need many connector types, easy replacement, or high long-term repairability.

Yes, a power bank can charge two devices at the same time when its circuit supports multi-port output. The total available power is shared, so each device may receive less power than it would alone.

A power bank may drain fast because rated mAh is measured at the internal battery voltage, not phone output voltage. Fast charging, large phones, tablets, heat, old cells, poor cables, standby drain, and low-quality battery cells can also reduce usable runtime.

Your power bank may charge slowly because the wall charger is weak, the cable is not rated for enough current, the power bank has low input wattage, the battery is large, the USB-C PD profile is not negotiated, or temperature protection is limiting charging speed.

You can leave a good-quality power bank charging overnight occasionally if it has proper protection circuits and uses the correct adapter. It is still better to charge it on a hard surface, avoid heat, and unplug it after full charge when practical.