Rubber mulch is made from recycled tyres. These degrade further with time leaching various chemicals. A cursory glance over the patent offices of the world will show you that a very large number of complex chemicals are added to tyres. Different tyres have different chemicals. Many of these are endocrine disruptors (hormone). If I had to choose between wood chips and rubber mulch, wood chips seems a more "natural" choice than pieces of old car tyres.