Mukhtalif al-hadith primarily concerns apparent conflict between two hadith reports. Mushkil al-hadith is potentially wider: a report's apparent sense may seem difficult in relation to a fundamental principle, another legal proof, or some serious interpretive consideration.
Not every difficult report involves two competing narrations. Nor does every pair of apparently conflicting reports remain problematic once context and qualification are clarified.
Responsible analysis begins by identifying the particular difficulty, then using evidence from language, chains, context, circumstances of utterance, and other sound proofs.