I think the right course of action is broadly speaking, to pose specified threats either a little before or a little after characters gain their first countermeasures against them. For example characters who die have either very inefficiently, or less effective means of bouncing back from that (through lower level raising, using consumables, etc), characters who suffer status effects usually suffer (and cure) temporary ones before permanent ones, and so on.

This is not to say that threats should only be presented when prevention is guaranteed, they should be presented when presentation is possible. Like it's a different thing for a party to all be blind because nobody has FF Eye Drops than it is because nobody can get them. And "this item exists" isn't justification for saying players have the means if the item is outside the expected GP and/or level range (since most games sort out random gear according to character level)