I thnk Tachyon even in theory is inpractical and should not be adopted too.
Unlike ordinary particles, Tachyon's speed is defined as "increases as its energy decreases." This should somehow result in a possible FTL state (Faster than Light) which is not possible, except for the so call -> Alien's warp drive......
And i dun thnk we wan a warp weapon (we are not talking abt notrium the freeware game here.) And we do not want something that is an 'ultimate research' for the FTL engine (where we can integrate into hybrid manmade ufo) into a 'mere weapon in arms'.
Hence, i thnk it was a good idea to reject Tachyon and adopt other techs like plasma, or even laser.