Nergal Pendragon
First Post
Yes, a perfect assassination is like rocks that you can't dodge. A perfect assassination is also only possible AFTER the villain knows who the PCs are, where they stay during downtime, etc. Villains should not just have that info by fiat; NPCs don't automatically know everything the DM knows.
In an RPG, a LETHAL attempt should have both a way for the PCs to spot/avoid it (in which case, they get XP) or a way that it goes awry (no XP, maybe they owe an NPC a favor now, maybe they suffer nonlethal harm... or at worst a reversible death). The attempt should also be an opportunity for clues.
Even in the case of the first statement in this section of your post, it shouldn't necessarily be like rocks you can't dodge. I do agree with you that the PCs should have some reasonable way of learning about it; they're the heroes of the story, after all.
I've had more than a few villains who've discovered all of that information, including even as far as what magic items the PCs possess a few times... but that doesn't stop the villain from either underestimating what is necessary to eliminate the party, not knowing about protections put in place before the villain started to gather information, or not knowing about a change in habits of the PCs. It also doesn't stop the PCs from finding out about the villains, or even finding out that the villain knows about them and what the villain knows before the villain has a chance to spring a trap.
I also like to drop hints on the villain searching for evidence, such as people the PCs are friendly with suddenly mentioning that someone has been asking about the PCs as of late, private investigators suddenly staking out shops the PCs frequent, maybe even family mentioning a friend the PCs have never heard of dropping by to talk to them...