Assuming that your subflow has multiple nodes in it then no it is not safe. As soon as the first node in the subflow has completed then another waiting node anywhere in your flow may run, using a round robin type approach. The result is that it is possible to have multiple messages running through your subflow at the same time. This is one of the major disadvantages of using context. It is usually much preferable to pass data in messages rather than using context.
If you cannot avoid context then a solution is to make sure that there is only one message in the subflow at a time. You can do this using the simple flow described in this post.
