Most telling quote about Iranian nuclear arms deal: Do you really want a treaty that depends on "goodwill" from both parties to work?

Here is a quote from the very end of an article in the WSJ:

. . . observers warned that given the complexity of the agreement, which contains one main text and five detailed annexes and totals about 100 pages, the risks of disputes over implementation of terms could cause delays or even derail the deal. 

“The technical obstacles can be surpassed with goodwill and diligence, but political hurdles can turn into poison pills,” said Ali Vaez, senior Iran analyst at Crisis International, an international conflict resolution group. 

“Neither Iran nor the U.S. has ever implemented such a complex quid pro quo. . . .

How good is a deal that depends on the "goodwill" for the two parties involved to make sure that the treaty works?