When have we ever seen Traya, or any other Sith for that matter, do this?
Traya and the Exile had a Force Bond. Traya does not have a Force Bond with ever one of her Sith (nor can she create them). Traya only communicated with the Exile while they were on the same planet, seemingly while they were within a few miles of each other.
How is Traya supposed to speak with such detail to Sith she has no special connection to if they're thousands of miles (at least) away?
Agreed, the Sith also aren't very forgiving of failure. Sending Force Messages would be more in line with Jedi, than with Sith.