Very poor. Totally biased against the English (they're apparently only wicked, bad, stupid, or bad and stupid) and by the third series the writers don't even attempt to hide their prejudices. The 'good guys' don't fare much better, with the Scots being portrayed as drunken, foul-mouthed, or drunken and foul-mouthed, the Irish are duplicitious thieves (who in dubious circumstances somehow find themselves at the centre of the action), while the natives are mostly reduced to stereotypes. The lone Americans (a male couple) are obscene and psychotic (whether they're portrayed this way because they're American or because they're men who like men, I'm not sure)... it goes on.
The show has some of the worst acting and attempts at British Isles accents I've ever witnessed, especially from the pneumatic blonde barmaid. The Captain Chesterfield actor has also been roundly criticised here... well, I'd say he did his best, what with the badly-written, cliched role he was given, but Cockney London, a difficult accent for any actor to master, wasn't the best choice for him.
As for Jason Moama's Harp? Are we supposed to be sympathetic to an entirely charmless, rude and violent man who indiscriminately and quite sadistically murders random people, just because they happen to be redcoats, or English, or simply in his way? (By the way, the British Army was neither made up of English alone, as they're portrayed here, nor made of purely incompetent sadists.) Are we supposed to believe that Harp is so mighty that he can kill dozens of armed men (who stand amazed while he kills their comrades instead of simply firing on him) without sustaining more than a few scratches? What a choice for a main character. Still, Jason did what he could with the script.
The good points? The scenery is beautiful, the set-pieces and costumes are believable (until Series 3, when most of the new characters look like they're wearing Hallowe'en costumes), and at least Lord Benton is a thoroughly entertaining and believable bad guy (until Series 3, when it's obvious the actor's given up on the show, like the better writers most certainly had) and Grace Emberley is a sympathetic-woman-in-a-man's-world who ended up the only character I still cared about by the end.
Having said that though, Grace really deserved better, as, frankly, did Sokodan, the native whose quest to save her people from white slavery was badly-handled, and the English barmaid character, whose transformance from former prostitute to eventual brothel and slave owner was poorly and unconvincingly conceived to say the least.
Honestly, having heard that Frontier was written by Canadians shocked me. With all the obsession over "freedom from the evil Brits", I would've thought that it was written partly by Mel Gibson, while the laughable video game-style violence reminded me of something from Michael Bay. I have some news for the writers: the British colonial efforts of the time were not something that were in fact supported by the vast majority of the British population, from the rich to the poor, as hard as it may be for some to believe, while slavery was seen as repugnant. The question of Canadian independence was, when it began to gain traction, strongly supported by the general public back in Britain.
Like many other reviewers, despite the above criticisms I enjoyed Series 1 and 2 (at that point most characters were still slightly more sympathetic and multi-faceted), but 3... well, it's no surprise why the show's been cancelled