Hi @Mak, welcome onboard! 
As you, Sitely for me is a standout and so easy to use! From my experience it has taken a different take on website building, adhering to the fixed-width responsive layout. This is what Muse had initially and then they changed over to fluid-width responsive and wasn’t that fun! 
Sitely has preset fixed breakpoints (and you have manual control over most of them), so when you increase or decrease the width of your browser it will show left and right margin until it hits the breakpoint assigned. But in saying that Sitely presents pixel perfect layouts everytime!!! 
For me I have gotten around the pixel perfect measurements between elements when I move them. After a while your eyes do the pixel perfect job.
I do like the idea to have the ability to stick elements to the edge of the browser window and I’m sure it is being considered by the Sitely Team. But creating an edge to edge layout is very possible in Sitely with a bit of thinking outside the box. Check out one of my posts on the subject with a working demo site to look at…