According to
the FAQ - you can't cancel a flight on Transavia made with miles, so they are effectively like Promo Awards.
However, they do allow you to change the date (no mention if there is an additional fee for this).
And despite the TicketSpy article posted last week, it doesn't appear that Dubai or Cape Verde can be booked....at least, they are not mentioned on the Transavia website...but maybe these are seasonal routes not currently operating?