CSS styling that was removed from OOjs UI core but is still used in (almost) every application uses to a point where I wonder, "Why is my app looking completely different from everyone else's." because they all use styling that I never knew about.

IMHO:

• object orientation (obviously the framework's hallmark, and likely the most maintainable way of doing things, but arguably not the easiest one to start with; thankfully a lot of documentation is now available)
• 'dirty' interaction between jQuery/DOM methods and UI objects (e.g. $div.append($button.$element))
• high frequency of changes to classes and styles (hopefully will fade out over time)
• MediaWiki-specific feel, lack of general-purpose features (e.g. numerical input fields)
• lack of clean, markup-based UI system (e.g. standard HTML + mediawiki.ui), again PHP/JS-specific feel

In T97264#1255071, @Ricordisamoa wrote:

• lack of clean, markup-based UI system (e.g. standard HTML + mediawiki.ui), again PHP/JS-specific feel

This might be addressed by T101666: Create parser tag(s) that render OOUI PHP widgets.

Regarding object orientation, it's hard to adopt one piece of OOjs UI. E.g. if you want a set of buttons that update the dialog they're in, you probably want to create your own widget whose constructor takes the button parameters, creates a SelectWidget that in turn does a .each on the button parameters to create new OOUIButtonOption, then on choose it emits an event that the dialog connects to some state update. "The answer to problems using OOjs UI is more OOjs UI" (Roan)

VisualEditor provides lots of examples of this sort of refinement of OOjs UI, but it's complex.

In T97264#1255071, @Ricordisamoa wrote:

numerical input fields

Addressed by T92263

I'd like to point out that while OOjs UI may seem awkward at first, magic happens when you start working with object orientation rather than trying to work around it. Documentation abounds, what is missing is a list of best practices users coming from other libraries may not be familiar with.