" Bogwood " sourced from a LFS that is correctly packaged Etc, will have been treated to prevent any microbes Etc which will be inside the woods structure from infecting the tank with potential diseases.
This wood is specially selected from mainly bogs that have a high content of rotted veggie material ( Peat especially ) and then sterilized using an Autoclave Oven, where the wood is baked for many hours under a low temperature to kill all the nasty stuff.
Hence the expensive cost of this product.
Any other wood sourced from just a place you walked the dog, will " potentially " have all kind of nasties inside it, and by using this kind of wood you run a real risk of harming your fish through an introduced kind of poisoning. @ is 100% correct in stating that it isn't the actual wood that reduces the Ph, but the tannins which are held inside.
As I previously stated the wood is sourced from bogs that have a high amount of peat content, and its this peat which the wood as absorbed through putrification that lowers the Ph when it leaches out.