Using scaffolding boards (new or used) for flooring or counter tops
I wouldn't recommend using scaffolding boards as a worktop in a kitchen. Most scaffolding boards in the UK at least are made of Tanalised (pressure treated) timber. Tanalith E 8000 is the trade name of the preservative used and it is not recommended for use where there is direct contact between food/water and the treated timber since it is water soluble.
Furthermore many scaffolding boards will be covered in concrete and mortar from their use which might have effected the longevity of the timber (wood doesn't like the highly alkaline and moist environment before the concrete/mortar cures). Any residual concrete/mortar will shed your planer knives so be careful. This is in addition to any nails or screw present.
Finally most scaffolding is flat sawn and may contain the heart of the tree. Since most boards are quite wide and long they may be unstable and warp especially if they have got soaked throughout their life by being sat out in rain. Video showing different types of cut/grain orientation
As in many things there is probably no one size fits all approach. The timber doesn't know what it was used for in it's past life nor it's future life. If the scaffolding boards are in mint condition, untreated and stable they should work quite well as a worktop. If they were sat out in the rain for the last 5 years and used by bricklayers 6 days a week then they probably won't be much good for flooring or a worktop.