Avery Mostly stuff like ergonomic advancements, stuff that cannot really be put down to one specific feature common to all modern belts. But there is a reason people buy belts from renowned manufacturers for the price of half a dozen generic chinese belts or more. And even those are designed so they could be worn for longer than the interval between two bathroom breaks (or the drainage in the shield would be quite redundant), but of course on those it might be just to tick a check box for a feature list or to entertain masochists - just because they can be technically worn all day for a short period does not mean that it is a pleasant experience.
But I am with you on the credibility constraints. Even with a relatively long break-in period, it seems a bit far-fetched to be wearing those things 23/7 for years. Even a month at a time with only short cleaning breaks and no real cool-down recovery period - especially since many of the girls here report having worn the belt at an age where I doubt their bodies have already completely finished growing. Slightly more believable with the boys who do not have to deal with the whole waist band and tight-fitting crotch strap, but with those I am surprised how few of them have had success at... worming themselves out, so to speak.
So yeah, plenty of things to be doubtful about, but wearing a belt on bare parts for more than a few hours is not really among them from my point of view. Now granted, we do not have the same anatomy, but for either sex there is a huge variance on anatomy in this area and some people might just have better luck with the cards they were dealt... if you want to call being relatively comfortable in a snug chastity belt for days at a time luck.