On the radio (TRX/TS) level, an SPDCH is the same as an FPDCH. The only difference is in the PCU. In the PCU, the GSL devices for FPDCHs are also fixed, while for SPDCH, they’re allocated on-demand.
From the PCU perspective, it’s more advantageous to use SPDCH instead of FPDCH, ‘cos it’ll use your GSL devices more optimally.
If your voice traffic is very high, then you MIGHT INITIALLY have high PDCH allocation failures when you activate SPDCH (same goes for FPDCH), ‘cos the BSC will not get any idle TCHs to convert to the PDCHs.
After some time (when the SPDCHs have been successfully allocated), then your PDCH allocation failures will purely depend the level of your CS traffic and the setting of parameters such as TBFULLIMIT or TBFDLLIMIT.