So at work I am one of the more experienced fellows, and I am constantly explaining to newer employees why we do some things a certain way - usually it's because we have looked into what they suggested and found that it wasn't feasible - my problem is how do I explain that in a way that's not condescending, but I don't want to encourage them in particular either, only for them to waste their time & end up with the same conclusion - it's not worth it / it won't work / etc. - but without being cynical about it or making myself seem like more of a pessimist than I already am.
Most of the time it's not a very clear-cut black-and-white as to why it won't be worth it, I can give a couple reasons, but for a full analysis that would take a lot of time and effort to dig up ALL the reasons (yes, some better documentation could help, but no-one really likes documenting dead-ends, but rather abandons them as soon as they realize it's a dead-end).