Marcel é muito enrolado, ele nunca cumpre com os seus compromissos.
-Marcel is not reliable, since he seems having no management system.
-Marcel rush from an activity to the next, so he never keeps an appointment/his appointments.
-Marcel is not that reliable, he doesn´t manage his activities properly.
-Marcel is unfocused, squander his time and then is always struggling with his activities/tasks.
-Marcel is not much focused and in control of his time, because of that he is don´t fullfill his appointments.
-Marcel doesn´t manage his time well, and it´s always struggling with his appointments.
-Marcel is always stressed and overwhelmed for not focusing in what has to be done.
-Marcel doesn´t prioritize his tasks and get himself overhelmed and overloaded.
-Marcel doesn´t take control of his tasks and projects.
-Marcel overcommit (doesn´t learn´t to say no), so he get himself overloaded.
-Marcel is too much complicated, are you giving this taks to him? ha ha ha.
-Are you sure that you want this job done, with Marcel in charge? (assuming that the boss already knows about Marcel´s poor handling of things).
Well, let´s wait some proofreading of my preliminary (and tentatively answer).
Hope it helps.