If your model predicts Tachyons is that bad?

[g.lemaitre]

When I first learned about Tachyons I was lead to believe that they meant your model had anomalies and you needed to go back to the drawing board and come up with a new model. But lately I've been hearing that they are not all that bad. I wish I had some direct quotes but I don't. Are tachyons anathema in the physics community? Do they automatically rule your model out if your model predicts them?

 

[Mark M]

They're very bad. Not as much the tachyons themselves, but the fact that a theory with tachyons predicts an unstable vacuum. This was the problem with the early bosonic string theories, which was resolved by including supersymmetry.

 

[Simon Bridge]

What Mark said.
Tachyons, and tachyonic fields are not exactly anathema - the math is kinda a mental exercise for theorists. But them showing up in a model that does not start out by presupposing them basically means the model is junk.

Presupposing them is basically saying you are deliberately making a junk model - but carefully. You may want to do this to expose possible flaws in the existing physics that may lead to new discoveries.