I'm often amazed at the number of actors who have made a very decent living, but who cannot do any accent other than their own regional one. Three examples: James Nesbitt (Northern Irish), Paul Copley (Yorkshire), Bill Paterson (Scottish) - they're the same in everything they've been in! Plenty of actors master all sorts of accents, but I always groan when I see any of the above in anything.
...one of the reason I struggle with David Tennant is his permanent accent. He can win as many awards as he likes, but I only ever see him as himself and rarely enjoy it.