There is always the possibility of a delay in getting the 421(a) benefit on new construction. If you close on the place prior to its approval you will pay taxes at the full effective rate until then and the abatement is not retroactive. I know of one building that started closings a full year before the abatment was approved; residents paid an average of $1000 per month in real estate taxes, and that $12,000 is not coming back.
But remember, you are not obligated to sign the contract once your offer has been accepted. Your lawyer should fully investigate the 421(a) situation as part of the due diligence procedure.