Yes. You do run the risk, however, that if the subdivision deed (obra nueva) contains errors, the registration office, to be compared with the Legal Security Service in Belgium, the registration of the obra nueva refuses. The promoter must then make adjustments to these obra nueva. Consider, for example, a correction of the number of official square metres of the flat.
In case the sale deed is executed before the completion of registration and if then the application for registration is refused for some reason, your title is based on a wrong situation. This erroneous situation must then be rectified thereafter with the cooperation of the promoter.
Therefore, we recommend signing the deed of sale only after the registration of the obra nueva was completed.