One of the main reasons that I'm not converting any of my components to Joomla4 is that I make extensive use of the Bootstrap2 responsive row and column layouts for view. Joomla 4 switched to Bootstrap 4 (or maybe 5) which has changed the way you do a grid layout using container, row and col-xx-N instead of row-fluid and spanN

.Some further musings on Categories and Tags sparked by my need for some of my components to use tags but sometimes to restrict the range of tags that can be selected. This started as a need to simplify the tag inputs for xbFilms and xbBooks where the same group of tags (eg Genres) might be used in both components along with other tags.

I'm a big fan of using joomla tags to classify items - articles, maps, films, books, people, reviews - and also using them to hold references and citations. Categories (in Joomla terminology) are also useful, but they have key limitations (which are advantages in some situations). Eventually the number of tags you are using across components gets unwieldy for the standard TagField form element to make it easy to remember and find the tag you want, whether using nested or Ajax mode.

Joomla database functions for building a query include a useful one for adding to the where clause with with a different condition (AND | OR) to the default you are using. 

But it has a gotcha if you are building a complex query with several possible WHERE elements