Signatures work perfectly well in the two usual cases: one, when the service is delivered properly and as expected, and two, when the service is not delivered as expected but no one disputes that both parties agreed to it.

Where they fall apart is where there's disagreement as to whether one party actually agreed to certain things, but that's not actually particularly common. Most court cases about contracts do not hinge on "did you actually agree to this whole contract?", they're about edge cases, or wording of the contract, or a clear breach that hasn't been dealt with yet.