How does a Holographic Pin stay in place?

Holographic Pins know to stay where you place them because the HoloLens scans your workspace and creates a unique environment based on that scan. When you place an asset RemoteSpark knows where it is placed within that workspace and remembers where you left it.

How accurate is the placement?

Assets are placed relative to spatial anchors that are created (invisibly) within your workspace. These spatial anchors are spheres with a 3 meter diameter. The HoloLens does its best to keep your anchor in the same space between uses of RemoteSpark but you can expect a little bit of travel (a couple of feet) over time.

Does having multiple devices active in the same workspace affect the accuracy of the placement?

It does, yes. Each device has its own idea of the workspace based on its own scans and that varies slightly from device to device. Device A may translate that data marginally differently than Device B and thus a Touchstone may have a slightly different placement from one device to the other.