Funny, we've just replaced DocuSign with a custom-built solution that will achieve full ROI in less than one year, thanks to recent price hikes on the contract.
I wasn't involved in the legal side and am not a lawyer, but in my limited understanding of US contract law (where this is exclusively used) it is quite simple to create a binding contract that will stand up in court. Building it, if anything, showed how simple the process can actually be.