I would not be worried about this style of hiring, it is much better than the traditional cold 'interview' with questions and answers because working at a startup is a two way street. Doing a project, internship or just contract work for a while before joining full-time gives the potential hiree a good sense of the startup he/she is joining. Its a small organization and therefore, its critical to make sure that you are joining the right team and its difficult to evaluate that just by a series of interviews where everything can be perceived as rosy.