Unholys ghul is a solid part of the specs dps but doesnt gain anything from expertise (Ghuls "expertise" is determined via the DKs hit rating).
Unholys playstyle has quite "conceivable" free global cooldowns in which you could just "re-use" the strike that was dodged due to low expertise (Runes for dodged strikes arent used). So all you effectively lose are dodged auto-attacks.
Frosts rotation is overall tighter and more random. Dodged strikes can lead to short times where all runes are active (wasting "refreshing time") which is a defacto dps loss.
Also, FS can be dodged (and you do not regain the lost RP, I guess) while DC cannot.