Damn, didn't know about that. I'll think carefully before installing one then.
Even with pollution, hazard may still be less than a 150% barren world. From a cost perspective, polluting a habitable may be cheaper than using a non-habitable world. (I do not like polluting habitable worlds, so I do not do that even if it is cheaper to do so, although I will exploit worlds that were polluted before I found them.)
Also note that after the forge has been installed long enough (about three months?), Pollution is permanent even after the forge is removed. If removed early enough, pollution disappears.
Sometimes, I build extra orbital works without forges on habitable worlds for higher production limit.
What do you mean "cannot be found" though?
In my previous run I found a corrupted one in some ruins / station. (I forgot which it was).
Pristine nanoforges are rare. If the player is unlucky, he may never find it in random salvage, and the only feasible option is to raid one of the big worlds for it, or perhaps visit the historian, but he rarely offers colony items unless player has all the blueprints. Having all blueprints is unlikely unless he raids the core worlds for missing blueprints, which if he is doing that to size 7 worlds, he is also capable of stealing the forge too.
Same deal with synchrotron. Rare enough (after the explosion of more colony items a few releases back) and required to meet demand for Military Base/HC that player may need to raid for it.