One phrase I've heard is deceptively simple, although there seems to be some debate about whether that refers to something that seems simple (but is in fact complex), or something that seems complex, but is really rather simple.
Six years ago, an online group debated this very phrase, and never really reached a consensus. You can read their debate by clicking on the link, but I think one contributor summed it up best by saying:
it's one of those phrases that can mean two opposite things, depending on context
The Free Dictionary has an interesting usage note on this deceptively complex word pairing; it shows that even experts disagree on the matter. Another blogger has warned, don't use this phrase in a vacuum; it won't be clear what you're trying to convey.
Sometimes, though, the intended meaning can be easily inferred from the context:
Golf is deceptively simple, endlessly complicated. (Arnold Palmer)
I'd say this question is deceptively simple, too – but I'll leave it up to you to figure out what I mean by that.