I also considered this, but then I remembered Kreia communicated with Atris telepathically - despite having no connections to her (as far as we know) - and that was over, what several thousand light-years? Traya was obviously somewhat unique.
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 every 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?
And then we apply that to our knowledge of telepathy as a Force power:
"Every Force user, Jedi or Sith, was able to use it. The range of telepathy itself is theoretically infinite, but the telepath must be able to sense the presences of those they wish to contact."
However we also have to consider the following:
"It should be noted that while every Force-adept was able to use it, very few were actually skilled enough to commit in actual telepathic conversations. Most Force-sensitive individuals could only exchange short four or five-word sentences to those that they shared close bonds with."
So while Traya is likely capable of tramitting more information, the Sith in question will find it difficult to transmit anything other that 'Help! We're under attack!" or something to that effect. So while Traya's forces would be prepared, they wouldn't know exactly what to suspect.
Sith aren't stupid, they don't forgive failure but they don't adopt the policy: 'If you fail, just don't tell me about it." We also have to consider that because of Malachor's electromagnetic interference, this is the only way she can communicate with her forces.
Agreed, the Sith also aren't very forgiving of failure. Sending Force Messages would be more in line with Jedi, than with Sith.