I know there are a number of disadvantages, but I do not think they're big enough to justify not implementing this method. The biggest drawback I can see is a bot/meso farmer uses your contact number and has it banned before you attempt to produce your account. If that's the case, there could be a fairly intensive way to clear a telephone number (it might need to be more intensive to be an effective ban).
Either that or you're out of chance, but the odds of that occurring anyway are slim to none to start with. Also, nexonNA could mechanically prohibit phone numbers from all countries outside of the serviced countries, thus making it even harder for chinese meso farmers to operate (they'd need a large supply of American/canadian phone numbers and corresponding telephones on hand to create more bots).
Ok, I know I have written about Hayato a few times already in the past, however I really, really believe that he's rather underpowered. He is, basically, a fast-paced mobbing warrior. But, that seems to be the only real thing . His 1v1 ability, Shinsoku, seems rather underwhelming, even with Hypers believed and even a decent piece of funding. Specifically, he's got no bind, silence/seal, or so on to counter things like Damage Reflect and Cancel Physical Damage. This endangers him at worst, frees him at least for many major bosses (Empress, Hard Magnus, to mention 2).
Aside from that, I think that Hayato, for it is current underpowered status, is quite hard to buff with out getting overpowered. It's already kind of a bully course early to mid game (Lv. 30-130) in terms KSing potential because of his mobbing abilities, plus his blisteringly fast strikes. If one were to adjust the damage%'s of his Sanrenzan, or perhaps add extra attack lines, it makes him overpowered. Perhaps if Shinsoku had a higher% damage per line, it can help matters, but I would suggest including a bind of some kind to his kit.
Now, abilities aside, I believe that there is a much greater reason he does not get as much love from players: his command. Granted, he has 60% command from the very beginning, but that would be it. Other courses get 80% control, maintaining their harm comparatively steady. Hayato? His damage jumps around as much as he does. There's also the issue of his attack multiplier (that is quite tough to adapt in a way that is balanced). https://www.mmogo.com/Maplestory-m/Mesos.html