Yes, UI library design is hard because it a series of trade-offs one has to make when designing the API. Make the API too customizable and it's a horror to use

Yes, UI library design is hard because it a series of trade-offs one has to make when designing the API. Make the API too customizable and it's a horror to use with horrible performances, make the API to rigid and people will complain it's hard to customize. WinForm is far easier to use than WPF... except that WinForm makes creating custom components a pain. WPF is easy to customize... except that it performs worse than WinForm, this isn't even up to debate... But also, all these moving parts are why a lot of developers chose to go with Web techs for their UI, for better or worse.