For ios : look at the number of various apis you can use to do widget positionning and animations : calayer, uiview spring n struts, constraints, physic based, and yet none compose well with each other.
IOS also doesn't have any good tech for offline storage ( core data should be burried once and for all) and swift relies on compile-time codegen hack to provide easy struct serialization, because techs like nscoding relied on obj-c runtime facilities and the language itself doesn't provide enough metaprogramming facilities to compensate.
Also, patterns like KVO are still found from time to time as a last resort in frameworks, while being famously unsafe and still rely on the obj-c runtime.
For android basically everything should be rewritten from scratch. Last time i managed a team of android developpers, we basically came to the conclusion that doing regular mvc while supporting a good market share meant recoding our own controller base classes and screen transitionning apis.
Also, patterns like KVO are still found from time to time as a last resort in frameworks, while being famously unsafe and still rely on the obj-c runtime.
For android basically everything should be rewritten from scratch. Last time i managed a team of android developpers, we basically came to the conclusion that doing regular mvc while supporting a good market share meant recoding our own controller base classes and screen transitionning apis.