I use gitg in a on Ubuntu VM, also known as "GitX" on Mac.
The basic UI of gitg is much easier to read and browse, the commits and details are top/bottom instead of left right. With SM, I have to expand the window a whole lot more sideways to be able to read the commits. Also with SM my name is under each commit.. but since I work mostly solo, it's added noise that makes the list more difficult to read (what with the commit messages and the author name underneath practically same font size. here the gitg top/bottom layout means the author names and dates can go in other columns making the commit list easier to scan.
In gitg I have a quick view of what files changed, how much they changed, I can dive in if I want to see the actual code. With SM it seems by default you get a full spread of all the changes across several files.
Lack of separation in general, lots and lots of text floating in space with few lines or separation. Lots of subdued color. Buttons that don't look like buttons.
That said my use case is fairly simple. I work mostly on local repos, so for me gitg is a convenient way to see what I was working on last time, and to stage changes in meaningful commits. I don't need it to do a bunch of other things.
The merge sounds great though.. I really miss that... but as it is SM lacks contrast and readability for me.
Hopefully constructive feedback.