I'm not sure why any viewer would compare a series set during American revolutionary times (the Civil War) to Downton Abbey. Downton Abbey was simply a remake of the popular (and recently remade) series, Upstairs, Downstairs. It was mostly an entertaining soap about a rich family and their household, but I digress...
First off, all of the actors are well-cast and do suitably good work. It's only been two episodes, so we'll have to wait and see as far as whether any heavy lifting will be required, acting wise. It's safe to say that we should expect to see the transformation of some of the characters, insofar as their beliefs are portrayed. There is a doctor, for example, who has no problem with slavery, but believes that it is his duty to equally treat Union and Confederate soldiers because, "Blood does not run grey or blue, it runs red." The nurse to whom he makes this argument (Nurse Mary/Baroness), is a staunch supporter of the Union and believes that as a Union hospital, Union soldiers can claim priority.
We have familiar faces in this cast - L. Scott Caldwell, Donna Murphy, Gary Cole, Tara Summers, and new ones in Shalita Grant, McKinley Belcher III, and Mary Elizabeth Winstead. While Mercy Street bears some similarity to The Knick due to its setting (a hospital) and a promise of change to come, it is less graphic in its portrayal of the everyday occurrences of the medical world.
With the popularity of Hamilton on today's Broadway stage, Mercy Street is a welcome addition to the TV lineup, as anything that encourages today's American to learn about the past is a boon, indeed.