As far as the law is concerned, I don't see why a clearly defined contract should not hold in law depending on what the application of it is. But I'm not a lawyer so that's not something that I can even speculate about given how absurd some of the loopholes over there are.
I thought the whole idea of the smart contracts is that they don't care about the law, the justice system can not turn them over. Contracts based on math, Honey badger don't care sort of thing, it either executes on the blockchain or not. What's the point of troubling with it if some judge can invalidate it?
But..but.. what's the advantage for the participants involved if a breach of contract or dispute is not defensible in court? Why not just spit and shake hands on a gentleman's agreement for that matter?
And anything that has legal precedent (99.99% of contracts) can be challenged in court. Smart contract or no.
Spit and shake hands as much as you want, brother, but only signature with your private key counts in the end!