We know more chemical and physical details than for other small bodies. We do not know how to use these details to constrain our models for comets. Abundance of gases in the coma is widely used to infer the ices in the protoplanetary disk. We do not know the relationship between coma abundances and those in the nucleus. Comets break apart under small stresses.
Nothing is known about variation of material strength with scale.
Deep Impact's primary scientific theme is to understand the differences between the interior of a cometary nucleus and its surface. Cometary scientists are convinced, albeit on the basis of little or no observational data, that the surface layers of the nuclei are highly evolved. We do not know whether the development of a mantle chokes off sublimation, thus trapping considerable amounts of ice in the interior, or if the mantle remains quite porous so that subsurface ice can sublime and escape freely. When the impactor makes a crater on the comet we will be able to determine dormancy or extinction. In the case of dormancy the impact will return the area to activity, whereas in the case of extinction no activity will be resumed.