My daughter is an actress. She says Norfolk and Bristolian accents are considered the most difficult to do but luckily she is Bristolian. An actor needs to immerse himself in a locality to pick up an authentic accent and luckily she has a good ear and friends with different accents. She records audio books and being able to assign different accents to different characters is helpful, but she will turn down books that she feels she cannot do justice to, a Geordie accent for example. Some listeners love her different accents and some are occasionally slated! It is really a personal thing.
Since I am Cornish, I loath some actor’s attempt at Cornish accents. Being from the county, I can often identify which part of Cornwall people are from as I imagine others can do if they live in say, Yorkshire. It is irritating when actors try to do a certain West Country accent and end up with a generalised country bumpkin sounding accent.
I love Sarah Lancashire but casting her in that drama, Kiera, I think it was called, set in Bristol just didn’t seem right. Better to have let her use her natural northern accent. People do move around to work!