Its only bad design if you assume the PCs should be able to overcome every obstacle put in their way. Personally, in most of my worlds, I just design things in a way that seems reasonable to me and its the PCs problem to overcome.
Thats all well and good if you, the DM, have provided them the necessary tools to overcome the obstacles they present.
If you've designed an adventure that requires Spell X to complete, but you've never given the party access to Spell X or (Magic Item Z that grants the benefits of that spell), you can hardly blame the party for failing to complete the mission.
After all, would Cap. Kirk be alive if the aliens hadn't given him sulphur to make gunpowder to shoot diamonds at the Gorn?
You could validly argue that "Arcanist #1 should have taken Spell X at level 6." However, the counter to that is that if Spell X was so important, your duty as DM is to inform the players know that someone would have to learn that spell.
If I was a vampire I jolly well would have a hard to get to coffin, especially if I was clever and intelligent and had hundreds of years to prepare it.
That makes perfect sense, but if the coffin is impossible for the party to get to, it makes the adventure pretty much unfun.