TL;DR: Headset cups are more play-safe, but the IS headset is simpler to 'fit', and though the mass manufacturing industry is as 'unsophisticated' as ever, most consumers don't care much anyways. It'll wear out eventually, but nothing much to worry about unless you do a hundred barspins every day.
My long gibberish:
From a layman's and manufacturer's perspective, it is the perfect standard as the consumers simply slip the bearings in and out they go, while the manufacturers don't get much flak in their machining tolerances as the demographic goes closer to being "always updating and upgrading" than "bike designs lasting a lifetime".
From a stingy engineering perspective, it has a few issues:
it's more sensitive to preload torque
mass manufacturers relatively care less for proper machining and tolerance (compared to decent aftermarket suppliers), leading to
non-standardized bearing choices, inconveniencing consumers, and
improper/misaligned/unflush/insufficient interface with the bearing's outer races, leading to spinning/walking/floating even on correct torque settings
Engineering principles mainly walk along "tolerances as tight as possible relative to purpose" so in that aspect, IS headsets can be a bit sketchy to look at.
That said, it's not that it doesn't work, it's just that it's a system that reflects more consequences when executed improperly. Just like pressfit bottom brackets, it's actually nigh-perfect when manufactured and fitted in the same nigh-perfect level of standard.
Headset cups are either the actual races (old style) or made so bearings are press-fitted to them. The softer cups gall to supplement the imperfections, turning it into a practically solid interface.
On a final note though, headsets rarely go full 360 degrees (except probably on trick, DJ, or BMX bikes), so there's more leeway and less to worry about.