Complex sentence: Often there are problems in identifying a given campaign as successful or otherwise: the answer may depend on the time-frame used, and on necessarily subjective judgments about what constitutes success.
Split into simpler sentences: Often there are problems in identifying a given campaign as successful or otherwise. Forming judgements about whether a campaign is a success or failure is inherently difficult: the answer may depend on the time-frame used, and on necessarily subjective judgments about what constitutes success.