Tachyons do not necessarily violate causality if you actually take the time to derive the laws of tachyonic kinematics consistent with special relativity.
Sure, tachyons will be strange (different observes won't agree on direction of travel or charge of a tachyonic particle - if we are even able to localize the particle at all; there will be frames of reference where a tachyon won't have energy but will still have momentum, ...), but not really any stranger that what we deal with in quantum mechanics and field theory, and the laws of physics will still hold in all frames of reference.
This answer doesn't deserve its downvote: The problems with tachyons were largely resolved in the 80s, based on work by Stückelberg and Feynman, Schwartz, Recami and others.
The community just lost interest in them because our current models of particle physics prevents bare tachyons:
QFT does predict real tachyonic particles. However, they are solutions of unstable vacua, which transit to a tachyon-free phase via symmetry breaking.
Stringtheory predicted tachyons as well, but got rid of them again when superstringtheory came around.
Tachyons are not gone completely, though: Virtual particles can be off-shell and thus tachyonic and feature in the description of self-energy.
Tachyons can also be useful to explain EPR-type quantum 'paradoxa' without having to give up reality.
Sure, tachyons will be strange (different observes won't agree on direction of travel or charge of a tachyonic particle - if we are even able to localize the particle at all; there will be frames of reference where a tachyon won't have energy but will still have momentum, ...), but not really any stranger that what we deal with in quantum mechanics and field theory, and the laws of physics will still hold in all frames of reference.