๖ۣۜI do sort of agree with your suggestion, but only to an extent, really. They are quite OP at the moment within my own opinion, but that will be changed when the races are modified and adjusted. There actually are some ways it is best to kill another race when you are different.
Dakkar is pretty simple, have a trap of lava for the non-Dakkar, and if the Dakkar swim through, have another trap that drowns them or crushed them. Not only does it incinerate the Dakkar, but it also suffocates the Fire-Resistance potions.
Naga, you can just run up to the enemy and poison them, no matter what. Undead don't have resistance from potions, so they choke and become ill very fast whilst you just swallow up the potion. If you are stranded inside a house, wooden/iron doors and windows don't stop the range of poison. Just throw it at the door and if they are close enough, they'll get splashed.
Undead killing an undead is pretty basic. Just haul ass into the arena and hope you have better armour, weapons and aim than them.
As I said, although I agree with your saying of Undead being OP, I won't support having it weakened because I am pretty sure they are going to be terrifyingly weakened pretty soon anyways.