I am not an expert on cybersecurity but undoubtly hacking and changing balances on one of those sites would be pretty easy in comparison on trying to do the same to an more established and regulated sportbook.
The security flaw would be even  easier to spot if the bookie was coded by some teen or colleage student by their own and did not buy the code from some provider or a professional programmer, for example.
Though, obviously, regardless on how unlicensed the bookie is, taking advantage of exploits like those is still a crime and nobody is supposed to do that and assume they would be doing some lesser evil, it would be a lie to themselves.

If you are asking these questions because you are planning to start you own unregulated sport book, then I don't have any further comments on it...