In the skutterudite compounds the anharmonic ``rattling'' oscillations of $4f$-guest ions in the surrounding ${\text{Sb}}_{12}$ host cages are found to have significant influence on the low-temperature properties. Recently specific-heat analysis of $\text{Pr}{({\text{Os}}_{1\ensuremath{-}x}{\text{Ru}}_{x})}_{4}{\text{Sb}}_{12}$ has shown that the energy of crystalline electric field singlet-triplet excitations increases strongly with Ru concentration $x$ and crosses the almost constant rattling mode frequency ${\ensuremath{\omega}}_{0}$ at about $x\ensuremath{\simeq}0.65$. Due to magnetoelastic interactions this may entail prominent nonadiabatic effects in inelastic neutron-scattering intensity and quadrupolar susceptibility. Furthermore the Ru-concentration dependence of the superconducting ${T}_{c}$, notably the minimum at intermediate $x$ is explained as a crossover effect from pair-forming aspherical Coulomb scattering to pair-breaking exchange scattering.