Mildly enjoyed it, but I lost my suspension of disbelief a few times. The biggest issue for me was the transition from the underdog to conquering hero seemed a little bit too quick and simple. Kentaro's search-engine-training-montage didn't really convince.
The other big thing was that Kentaro's internal model of his own competence seems out-of-whack.
If he was a "superhacker" then he would know that, without having spent significant time and effort in-prison (somehow), or since his release, he would be massively out of date. I used to be pretty good at that security side fifteen years back, but I've been focused on other stuff since. I would never in a million years think I would be competent to jump back into that arena without some remedial learning first. The technology and communities around the field evolve too fast. And I'm still "in" software development on a daily basis and haven't been locked up for thirty years.
So if Kentaro was a superhacker, he would never have gone into that initial meeting so unprepared and with such a poor understanding of his own skill level. If he wasn't a superhacker he wouldn't have been able to get "good" fast.
He might have gone in to that meeting desperate, not having an alternative, knowing he would likely fail, and hoping for loyalty from the old firm… but that wasn't the vibe I got from that initial meeting.
Other things that niggled for me.
* The SFnal elements felt tacked on to me. You could take a few years off the setting, change mom's illness, and the story would work just as well. Better even, because…
* Even with just the few extra years in the future the story is set in, it seems unlikely to me that the phone wouldn't have some kind of biometric security on the 2FA. Several of the devices in my home have fingerprint recognition right now. Why does the status conscious mob-boss have a retro phone? :–)
* In the same way it seems really unlikely that any sensible mob boss would be doing stuff on their home desktop. Criminals are already getting wise to things like key loggers — and this is another five/ten years in the future. The level of opsec for a major crime lord seems… poor ;-)
* I can't think of any international airport I've been in where I could have a reasonable chance of inserting myself into a specific part of the security queue. The plan relies on luck far too much.
* In the timeline, if Kentaro was in computer security, he would have been familiar with rainbow tables. They've been talked about since the 1980s. SQL injection attack since the 1990s. Again, Kentaro's level of competence and knowledge seems off.
None of these things completely ruined the story for me, but they kept popping me out of it so I could see the machinery (as it were).