Really depends on the land in question. Some forests have thick underbrush, some are almost completely clear. A lot of undeveloped land isn't even forest - swamp, desert, rocky mountains. Sometimes it's far easier to navigate than a parking lot, IME.

Very true. Bordering a big block of public land doesn't mean you can walk through it easily, and the acreage number says nothing about brush or deadfall. That's part of why the site only reports what it can measure (the shared edge, the size, whether a mapped road reaches the parcel) and doesn't promise anything about getting around once you're out there. Trail data would be a nice addition someday.