Here are a few issues I can mention with the mix.
1. Aggression-Angels tend to be more aggressive and will oftentimes bully the discus. Furthermore, angels are often eager to breed and can be a problem in a community.
2. Lion's share-Angels are greedy pigs whereas discus are grazers taking their time to chomp on the food. Make sure both are getting their fair share or at least, the discus must.
3. Diseases-Angels are often more resistant to diseases than the discus. As long as you are maintaining your tank well, I don't see why you should worry about it.
Most of us have kept them together successfully but it is down to luck whether you mix them successfully or not.