I don't think we know enough about Eighth Implement either but from what we do know, I think it qualifies as far as what we do know. Ea was made before the concept of a "sword" but I am unaware of it being made before the concept of a weapon.
I don't think gae bolg is needed per se to refute bastion. It is just the most extreme example. The threat of bastion is there but that threat is small due to tradeoffs. I look at it like this. Every time any big attack, be it a non NP, a C rank NP, or an A rank NP, the choice has to be made to expend the prana to use Bastion or not. If it is an A rank, it is more worth it to use bastion. If it isn't, then using it entails spending much more prana than the opponent did at which point, damage might not have been done but a huge advantage was gained. The price paid just by using it is heavy. I look at it like an ability that turns a lethal blow into one that gets you in an uninjured appendage. You use it once and get your off arm disabled. A disadvantage. Use it twice and one of your legs goes down. Another disadvantage. Etc, etc. The prana disadvantage is about that important. One doesn't need to actually kill the servant outright. Making him spend all his prana or so much of it that he can't effectively fight anymore is as good as winning. If anything, a defensive NP is generally most important in usefulness based upon its flexibility and bastion is not flexible.
I'm going to stop now too since I think Akashi wouldn't terribly appreciate me breaking down more of a full guide on how to make bastion useless.
