The worst case with using too low a charging rate is that the charger will not effectively detect when the cell is fully charged. In that case, the charger continues to charge the cell, it is overcharged, and can be permanently damaged. Unlike the old NiCd cells, NiMH cells don't tolerate overcharging well.

A higher charge rate will provide a more definitive signal to the charger when it is full, as well as recharging in a much shorter time. But that's a balancing act, too, because higher charging rates means a hotter battery and heat can damage the cell.