Tachyons, or at least ones with real mass, look impossible to me given relativity (I can imagine tachyons of a sort without real mass but these could not be used for engines and weapons), and this seems well supported by many physicists; I remember one (Jim al-Khalili) saying that he would eat his underpants live on television if any particle with mass went faster than light. None of the theoretical engines I have seen yet seemed to rely on tachyons (the Q-drive was based on some kind of fluctuations in vacuum energy while the Alcubierre drive warps space itself).
I would not really call it a "hard" science explanation, since it relies on some very dubious ifs, but it does not at all smell of bullshit.
Science fiction is about things which are possible in the vaguest theory, in that case.
Divorcing appearance from utility is a bit silly for role playing purposes or even gameplay itself. Take DF for example. If a dwarf grabs a goblin's helmet and tugs it off him, then the goblin's head is bare and can be smashed with a *steel war hammer*. But if the goblin's armour was invisible, and he only appeared to be wearing a loincloth, the dwarf could not do this, or not as easily anyway, and might not think to do it, instead striking the goblin's invisible armour to little effect. If the game is multiplayer, it could be even worse - imagine shooting an arrow at an apparently naked enemy only to find that she was wearing an invisible cuirass!