Customisation and flexibility is good. That's why people like Android and hate iOS.
Bloatware and fragmentation is not. That's why people hate Android and like iOS.
A theme engine is absolutely necessary. So if they want, people can turn a Samsung UI to a stock-like UI. But on the other hand, the stock Google Android doesn't have a theme engine. Hence you are essentially stuck with the same icons.
That said there is no one single best skin/launcher/UI. For example, although many including me prefer stock-like design, some other skins do offer better and useful functionality much before it is baked into pure Android (eg: Multi-tasking, Clip-board, Picture in Picture, Dual Spaces App, etc.)
Among the given options, I am fine with almost everything except Huawei's EMUI and Xiaomi's MIUI (I mean just look at the pre-historic call, contacts & camera icon of EMUI). While they shouldn't sacrifice the rich functionality of their UI, they should avoid the iOS-like Chinese style UI and should adopt the stock-like look that Lenovo has started to embrace.
Lol I thought they were talking about custom launchers downloadable through playstore well sonce its skin I like LG and Samsung most than comes Sony and Moto
Launcher and skin aren't same WTF! Nova is one of the best launcher, Stock android launcher is dull and stupid, missing important customisation options, but obviously the only android exists in my dictionary is the stock android skin/framework!