The phonon anomalies and their relationship to the structural inhomogeneities in Pb-based relaxors are studied on the basis of stoichiometric PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 and Nb-, Sn- and Ba-containing PbSc0.5Ta0.5O3. The ferroic clustering and the development of the ferroelectric state can be followed by analysing quantitatively the intensity ratios of the Raman scattering arising from the corresponding local structural distortions. PbSc0.5Ta0.5O3 and PbSc0.5Nb0.5O3 differ from each other in the length of coherence of Pb shifts from the O layers, which leads to dissimilarities in the ferroelectric state. The incorporation of additional elements in the cation positions heavily influences the incipient ferroic species and restrains the formation of proper ferroelectric state, thus favouring the non-ergodic state. The partial substitution of Ba for Pb and Sn4+ for the B-type cations induces additional local structural deformations consisting mainly of BO6 distortion along the three-fold and four-fold octahedral axes of symmetry, respectively.