I'm assuming he means that the "+x% damage with one-handed weapons" armour enchantment does not work correctly with daggers.
I feel daggers are left behind quite a lot in the one-handed mastery tree anyway. You're better off ignoring One-Handed and just getting the 15x sneak attack perk from the Sneak tree.
Yes, that perk when combined with power attack delivered from the right range (which requires some finesse) works quite well on most opponents. I am not certain if it will ever work on dragons, however. And in general it's pretty difficult against anything with high awareness. So those battles will presumably require some other subterfuges. Fortunately, the main campaign seems to be designed with significant options for non-combat characters.