Short work sample tests don't need to be simple work sample tests. One of Matasano's work samples required candidates to reverse-engineer a binary protocol, build their own client for it, and then use that client to find vulnerabilities. If you had no aptitude or experience with that problem set, finishing it would take a very long time.†
Ultimately the problem you're really describing is the challenge of hiring people you don't really know how to hire. If you know very well how the role you're hiring for is supposed to function, you should be able to generate challenges within whatever constraints your candidates have. If you don't know very well what the capabilities of that role need to be, no interview process is going to work reliably for you.
† (we didn't care, by the way: if you did well with that work sample, it mattered very little to us whether you were prepared when you started it).
Ultimately the problem you're really describing is the challenge of hiring people you don't really know how to hire. If you know very well how the role you're hiring for is supposed to function, you should be able to generate challenges within whatever constraints your candidates have. If you don't know very well what the capabilities of that role need to be, no interview process is going to work reliably for you.
† (we didn't care, by the way: if you did well with that work sample, it mattered very little to us whether you were prepared when you started it).