How to build the workflows is not apparent. Even the books out there published on SharePoint Development or Administration using SharePoint Designer tell you minor things; how to start, how to stop, how pause, etc., but nothing really defined as a Standard Best Practice to build (Insert Business Process Here) workflow. For example, a detailed explanation of how to use the action "Start an approval process". There are 3 different states in this workflow, and there are so many options in each state, and there is not a clear path. That is just one out of the box action that has 3-4 Levels of detail, without any direction. Your only hope is Google Searches, YouTube Videos, and if you are luck, there is a local college or university with Computer Education Training that focus on specific specific programs and teach you the in's and out's of the program, such as the University of Missouri - St. Louis. They have an entire division dedicated to the use of these types of applications such as SharePoint Designer, with actual Certificate Programs, of which I obtained (http://www.cetc.umsl.edu/). The classes there on SharePoint Designer gave me the knowledge to become the Architect and Designer I am today.
The number of actions available out of the box are limited. Granted the 2013 platform workflows do have a larger number of actions available, but still they are limited. One of the things I have done is develop custom workflow actions for SharePoint Designer. Sadly this only works with SharePoint on Premise Systems and you have to develop it in Visual Studio, and that program is as intuitive as learning a new language. The workaround I have used to add nearly 40 additional actions, with AMAZINGLY DETAILED INSTRUCTIONS with examples is the Workflow Actions Pack by Plumsail.com. This $500 annual investment has elevated the level of development with SharePoint Designer to outer space. The best part, it is not difficult to use, and makes creating workflows exciting.
The absolute worst thing about SharePoint Designer is that Microsoft is not releasing another version after the 2013 version. Microsoft will support SharePoint Designer 2013 for SharePoint 2013 & 2016 on Premise systems until 2026 (https://blogs.office.com/2016/01/20/sharepoint-server-2016-and-project-server-2016-release-candidate-available/). However, for the SharePoint Online (Office 365) environment may not support it for that amount of time. Microsoft may cut the time shorter due to SharePoint Online being a Multi-Tenant Environment.