Kanican did testing awhile back on the exact hate values of pretty much everything. Add effects generate no hate.
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Enspells overwrite additional effects from weapons. Best example - Twilight dagger - you'll allways overwrite the additional effect.
Samba effects will not however take precedence over add effects of weapons.
Samba effects only appy to the mob and not to the weapon.
It's right, you put the effect on the mob with a direct melee hit but you can apply the effect with any melee weapon even if you allready have an en-effect on your weapon (spell or natural).
So the priority would be en-effect > weapon natural effect > samba