Bonding Coat does not stand up to damp conditions. Therefore if in a damp environment sand/cement is safest. However it depends on age of the house (levels of insulation and ventilation that are present). So if there is no background dampness from poorly fitting windows and walls plus no excessive condensation from windows and walls due to adequate ventilation etc then bonding coat will be fine. In the shower area it will be fine if your tiling is good. In my opinion wether there is sand/cement or bonding coat may be immaterial as if the tiling lets water in a bit of soft plaster may be the least of your worries so just make sure the tiling is good and you should be fine!