For the last 18 months I have been working full-time on Drupal workflow solutions, first with the Deploy suite of contrib modules, and now with the core Workflow Initiative.
It turns out core is a lot harder than contrib. Everything we have mapped out in the plan for the Workflow Initiative is already done, and working, in the contrib modules. So getting it all in core should be easy, right. Just port over the code from the modules into core APIs or core modules and it’s done. Well… no.
Everything in core needs to be generic, it needs to work for everyone. In contrib you can choose not to use module X if it’s not compatible with module Y, or if it doesn’t work with your use case, in core there is less of that luxury.
Also in core everything has to be done “properly”. We can’t just get a solution that works well enough and run with it, we need to be sure it is the best (or least worse) solution.
Some examples of the issues we are facing so far is with the Upgrade path between revisionable / non-revisionable entities issue. In contrib we use the migrate module to migrate all content out, make entity types revisionable then migrate everything back. For core this is not stable enough so we need a custom set of update hooks. Then Add StatusItem field type to store archived state in contrib we just add a new base field to all entity types to store the archived state. In core there are performance concerns with a new field and new conditions.
Thankfully there are so many people working and thinking about things that will help some of these areas, and as we continue to develop core we should be able to make things easier.