It might have a slight acidifying effect. If you really want to increase acidity, you will need to apply sulfur or ammonium sulfate. Do not apply aluminum sulfate. Aluminum is toxic to plants.
Rotten wood has some nutrients, including NPK, Ca, Mg and trace elements, but they are not present in high concentrations. They might be enough for heathers, but you would need more fertilizer if you were growing vegetables, for example. By the way, you don't need to worry about applying a little Ca and Mg to acid-loving plants. All green plants need some Ca and Mg.
I think the best feature of rotten wood is that it is humus, and humus does wonderful things for soil structure and moisture and nutrient retention. It will also encourage earthworms, so you have to decide if that's good or bad for you. The bad part about humus is that it eventually disappears and needs to be replaced. It lasts about two years, unless your climate is very cool. If you have plenty of rotten wood, you don't need to buy the ericaceous compost.
You will also discover that rotten wood is a wonderful medium for germinating seeds, including weed seeds. That's why people use bark or wood chips. They suppress weed germination.
When you are harvesting your decayed wood, watch out for carpenter ants, termites, borers, etc. I would closely check for bee and wasp activity before digging into the stuff.
