Mind if I add something in Donnel's defense?
Most of us are probably going to turn Donnel into a mercenary first chance we get. But when compared with Severa, Gregor, and Inigo, his Luck growth is so off the roof it doesn't compare to the other three pure mercenaries, and certainly tops them for availability, even counting the time it takes for him to reclass.
You know what runs off the Luck stat? Armsthrift and Despoil, or more to the point: Leif's blade which is only a D rank and also has as much crit as a killing edge and you get for free with renown. In all my play throughs, I've only had his original Leif blade break once. (And by forging mostly crit and earning Sol, he certainly becomes a very profitable and durable one man murder machine.)
Yes, Donnel does take a little investment, but I find he certainly pays back in the literal sense with interest.