Well, From a metallurgical viewpoint bog iron was never ideal, Silicates and phosphates are very hard to remove, and would require a lot of purification.
Todays purification methods varry, but include melting(Due to the invention of coke) powdering and processing with chemicals that affect the impurities and not the iron, and other such nonsense.
Back then, purifying iron into wrought iron required literately beating it out of the surface, bog iron would obviously take longer than mined iron due to the impurities, and also, due to the massive amount of energy and effort required it isn't a viable source of iron in large quantities.
If you want surface ores that are viable, and realistic, mining copper out of surface rocks burried in the lines of hills ingame wouldn't be far off, or the always suggested panning for gold.
More information here:
viewtopic.php?f=5&t=35286