I would MUCH rather not have a unit at all than build one that blows up randomly. That just sounds like a very frustrating, and not-fun game mechanic. Just because they were around doesn't mean that they need to be represented in the game if they didn't play a major historical role. Civ doesn't need to cover every single historical type of unit that ever existed, or else it'll be flooded with all sorts of useless units. Or worse, all-too-useful units lacking a counter strategy that dominate all of those other units that actually DID play a role in history.