In travelling and staying for extended periods in southern and western France, I've encountered difficulties in communicating with the locals, and even some native French say they have difficulties communicating with their compatriots. This seems to go far beyond the langue d'oc/langue d'oiel divide. Won't go into it now where I've heard/seen French people say they don't understand the locals (or in a case of a friend here, her sister picking up an accent from a tow 60 km away).

I've found it easiest to understand Bretons because they were pretty much forced to learn French within the past century so it wasn't a proto-French dialect that was in use then. Similarly, Corsicans are somewhat easier to understand as they used to speak proto-Italan dialects (there seems to be a divide in the island between those in the far south (Bonifacio) and everywhere else in the island).

I've found one of the most difficult in traditional France is along coastal Charente were a strong dialect exists. One of the easiest has been in small towns in Gascon where the regional dialect is to pronounce everything clearly (such as the final e).

In the south, I've normally encountered people from other parts of France so not too many "natives". One other region which is difficult is the SW part where people speak Catalan or a Catalan-influenced dialect.