Oil sands form naturally over time as is, so yes they are safe for the environment. The build up and pack together as time moves forward and oil leaks out of certain parts of the world, so since they are naturally forming most of the time then they are part of the environment.
There are not major concerns with oil sands. Environmental groups always make a big deal of everything, however generally there is no concern. Similar to natural gas fracking, where environmental groups are up in arms trying to raise lawsuits, there is just no hard evidence to support claims that it is dangerous to the local population, only concerns. While oil can be dangerous to the local population, this is not the case when properly managed. Oil sands are safe for the local environment.