I feel that WCV's are weaker than many character options because they don't have one focus area and have a sort of multi-skill dependency problem (similar to multi-attribute dependency from D&D, here's looking at you Monks/Swashbucklers/Hybrid-PRC's). They need high Deceit/Intimidate for their Incite Emotion, high combat skills to make full use of their Strength/Speed powers and Discipline so they don't get overwhelmed by hunger every scene. They also have this mix of physical combat powers and mental manipulation powers that make them less strong in either area than a character focused only one one.
On the other hand I don't know why you say they "burn themselves out in conflict," and that they are "one note" in role playing. They are one of the character types that I feel does best in a long fight. They do not have limited resources like Wizards, they have recovery/toughness if they need it, they generally have high defenses due to the need for discipline and their speed power. On the other hand their attacks are weaker than a Wizard's or a character built for physical combat (like a shapeshifter whose were-form is combat oriented).
I also feel they have some of the more interesting role playing. They have all the potential decisions of any human-like character plus the added depth of a struggle with their demon. If you let them just feed on every normal person they meet with no down side then they are one note, but if you produce consequences for killing people (like trouble from the local authorities, or just crises of conscience) then they have to be very careful with their powers and their feeding, this is an area ripe for compels. Like any character it just comes down to how you play it, if you play them "chaotic neutral/evil" then they can be one note (unless you make this a complication for them), if you try to play them "good" then they get a lot of depth from the inner conflict.
