Oh, don't get me wrong, I have no expectation that this rollout will go well, at all. Y'know all those cautionary tales about throwing technological solutions at human problems? Well, Nigeria has a hell of a lot of human problems and I doubt that the technological solutions will work better than usual.
My observation was narrower: overt violence, imprisonment, or other radical life disruptions among debtors are symptoms of an immaturesystem of credit and interest extraction. Now, since large parts of Nigeria have symptoms that suggest such a system(high levels of local corruption, massive inequality, minimal social infrastructure and documentation, etc.), it is likely that such relatively primitive debt mechanisms will persist, at least for a time.
However, the cards are unlikely to be the ideal mechanism: electronic transactions are traceable, to a degree, and the only greater enemy of corruption than an honest man is a dishonest man's boss who is wondering why he isn't getting his cut of the take. This(as much as any rule of law) helps discourage excessively overt bribe-taking. There's always a bigger fish you'll have to pay off if it becomes clear that you are worth targeting. Ground-level corruption will prefer to deal in cash, or some 'cash equivalent'(if I had to bet, it'd be a local cigarette or phone top-up card, or some other item that is both intrinsically useful and relatively nonperishable if purchased and returned to local vendors: people who need to pay bribes will buy this good, people receiving bribes will receive them 'in kind'; but be able to return, likely minus a small restocking fee, any excess that they don't need. All electronic transactions legitimate, works even if cash if phased out, still transfers value. Something like what happens in prisons with commissary goods).
As for the rest, I stand by the basic conclusion: brutalizing your debtors is inefficient unless there is no other way of keeping them from escaping, or they are so helotized that their value as slave labor is about as high as it could be. Maximum efficiency is somebody able to live a 'normal' productive life while desperately paying the minimum balance on their credit card every month. (Note, I don't say that this is a good thing, it bloody well isn't; but it's the ideal and most advanced form of interest extraction. All other methods, even if scarier or more theatrical, are less efficient and ultimately contrary to the interest of the lender, if they can achieve this one.)