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A relativistic FTL universe would be acausal (magical perhaps?), a causal FTL universe would have to assume (against the weight of all observational conformation) that Einstein got it wrong, and only Newtonian physics applies.
In addition, wormholes are the only form of FTL that don't conflict with causality (technically a wormhole is a local distortion in spacetime, it is not true FTL at all).
FTLwarp drives don't preserve either, which is a good reason to think they aren't possible at all.