Not a lot comes to mind that really falls between “basically trivial” and “you’re gonna need to hire a team of PhDs” or “you have accidentally asked me to write a custom distributed file system, and you probably don’t want to pay for that”

Which is to say that usually the hard stuff is an accidental request that nobody actually wants to pay for, so the only thing “hard” about it is recognizing that the client/stakeholder has wandered into territory they really ought not.