I do not get the bad reviews? It's a good show! Might there be some racism rife amongst the reviewers? It's the only reason I can come up with as to why people are trashing this series.
The whole series has a play-like feel to it, with beautiful sets, costumes and lighting. Its exploration into emancipation and the difficulties faced by Lincoln's government are unsettling. I couldn't help but feel sickened and frustrated by the people of the era - more so as the echoes of the time can still be seen in present day America. It's not as horrendous and gut-wrenching to watch as 12 Years a Slave, but it's enough to make your blood boil. The show strikes a balance between exploring the political and ideological upheaval of the era, and the human trauma and cost of such change.
The acting is great, but I agree that the plot is a bit slow. I think it depends on what you are expecting. If you are interested in seeing the politics and changes of the era, around the story of the capture of Booth, then you'll enjoy it. If you are looking to watch a literal 'manhunt' then you'll be disappointed.
Perhaps the shows name is really the problem?!
The main complaint from other reviewers seems to be that the actors don't look/sound like the real life people. 'Stanton had a beard!' 'Lincoln didn't sound like that.' Which, in my opinion, are ridiculous condemnations. The show is broadly historically accurate, but it is also a TV show adaptation. If you want to get a 100% factual version - read a history book.